Integrated optical control element and a method for fabricating the same and optical integrated circuit element and optical integrated circuit device using the same

ABSTRACT

The integrated optical control element of this invention includes: a first waveguide for allowing light incident from outside to propagate therein; a multilayer structure for allowing the light which has propagated in the first waveguide to be incident thereon and for transmitting the light therethrough or reflecting the light therefrom, the multilayer structure including at least one layer having a refractive index different from an equivalent refractive index of a region with which the at least one layer is in contact; and a second waveguide for receiving at least part of the light transmitted through or reflected from the multilayer structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an integrated optical control elementused for a optoelectric integrated circuit and the like required in thefields of optical transmission, optical measurement, optical informationrecording, and the like. More specifically, the present inventionrelates to an integrated optical control element with a small variationin reflectance and transmittance in response to a variation in operatingtemperature and light wavelength used, and a method for fabricating thesame.

The present invention also relates to an optical integrated circuitelement where the integrated optical control element and an opticalelement are directly and integrally formed on a single substrate, and anoptical integrated circuit device where the optical integrated circuitelement is combined with a further optical element by mounting and thelike, or an optical integrated circuit device where the integratedoptical control element is combined with an optical element by mountingand the like.

The optical elements as used herein include, in addition to theintegrated optical control element, a laser diode (LD) for emittinglight, a light emitting diode (LED), a photodiode (PD) for receivinglight, a modulator, an optical amplifier, a microlens, a passivewaveguide structure, and the like.

2. Description of the Related Art

In a waveguide-type optical integrated circuit element, an element forcontrolling light is indispensable, as well as a light emitting elementand a light receiving element. In particular, a multi/demultiplexer forcoupling/dividing light is an important integrated optical element inthe construction of an optical system.

Conventionally, as an integrated optical control element used for theabove purpose, a multi/demultiplexer using a Y-shaped branchingwaveguide 1000 as shown in FIG. 18 is known (H. Nishihara et al.,"Optical integrated circuit", Ohm, 1985, pp. 41-45 and 265-266).

A directional coupler using the effect of weak coupling of light betweentwo waveguides 1901 and 1902 formed close to each other in parallel isalso known (H. Nishihara et al., "Optical integrated circuit", Ohm,1985, pp. 45-47, 51-54, and 266-267).

A trench-type element as shown in FIGS. 20A and 20B is also underexamination (Japanese Laid-Open Patent Publication No. 4-151886). FIG.20A is a plan view of the element, and FIG. 20B is a sectional view ofthe element taken along the line X-X' of FIG. 20A. In this trench-typeelement, a deep groove (trench) 2002 is formed at a crossing 2001 ofwaveguides by etching so as to change the refractive index locally andthus to effect partial reflection of light.

A star coupler as shown in FIG. 21 is well known as an opticalintegrated circuit element for light multi-branching which branchesinput light into eight parts. This star coupler has a structurecombining the elements shown in FIG. 19 in series.

The above-described conventional integrated light control elements andoptical integrated circuit elements have the following problems.

(1) In the Y-shaped branching waveguide-type multi/demultiplexer shownin FIG. 18, the angle of a Y-shaped branch 1100 is required to beseveral degrees or less in order to suppress radiation loss.Accordingly, the entire length of the element is 1 mm or more, which isextremely long compared with other integrated elements. Such a longelement is inconvenient for application to an optical integrated circuitelement.

(2) In the directional coupler shown in FIG. 19, the coupler needs anoptical coupling length of about 2 to 3 mm in order to function as acoupler. Moreover, as in the case of the Y-shaped branching element,since the curve angle of the curved portions of the waveguides leadingto the coupling portions thereof is required to be several degrees orless. Accordingly, the entire length of the element is as long as 5 mm.Such a long element is inconvenient for application to an opticalintegrated circuit element.

Another disadvantage of the directional coupler is that, since theallowable range of wavelengths for the element is as small as ±3 nm andthat of operating temperatures is as small as ±5° C., a mechanism forsuppressing the wavelength variation of a light emitting element in anoptical integrated circuit element and a mechanism for keeping thetemperature of the entire element constant are additionally required.This makes the entire system complicated.

(3) In the trench-type multi/demultiplexer shown in FIGS. 20A and 20B,the reflectance of the element greatly varies with a small change in theposition, depth, and width of a trench 2002 by only about 0.1 μm. Thislowers the production yield and thus reproducibility of the element.Another disadvantage of this element is that, since part of thetransverse-mode of light propagating in the waveguide is reflected, thecoupling efficiency of the reflected light and the transmitting light toan output waveguide is low, resulting in large optical loss.

(4) In the star coupler for light multi-branching shown in FIG. 21, theentire length of the element is as large as about 100 mm due to thereason described in clause (1). This makes it complicated to handle theelement, as well as making a system using this element large. Otherdisadvantages of this element are as follows. Since output waveguidesare only arranged on one face of the element, fiber-connected outputs ofthe waveguides, each having a size of several millimeters, are requiredto be arranged on one end face with a narrow space therebetween. As aresult, the width of the element is large (40 to 50 mm), freedom in thepackage design is lost, and the connection of fibers to the outputs ofthe waveguides is inconvenient.

SUMMARY OF THE INVENTION

The integrated optical control element of this invention includes: afirst waveguide for allowing light incident from outside to propagatetherein; a multilayer structure for allowing the light which haspropagated in the first waveguide to be incident thereon and fortransmitting the light therethrough or reflecting the light therefrom,the multilayer structure including at least one layer having arefractive index different from an equivalent refractive index of aregion with which the at least one layer is in contact; and a secondwaveguide for receiving at least part of the light transmitted throughor reflected from the multilayer structure.

In one embodiment, the multilayer structure includes two or more layershaving different refractive indexes, each of the two or more layers havea thickness expressed by:

    (2m+1)λ.sub.0 /(4n cos θ)

where λ₀ is a wavelength of light in vacuum, θ is an angle formed by alight proceeding direction and a normal of a layer, n is a refractiveindex thereof, and m is an integer more than 0.

In another embodiment, the mode of the light incident on the multilayerstructure from the first waveguide is a TM mode, and the angle θ is 45°or more.

In still another embodiment, the first waveguide includes a first corelayer which becomes a core for guiding the light, and a first claddinglayer located below the first core layer and having a refractive indexsmaller than that of the first core layer; the second waveguide includesa second core layer and a second cladding layer located below the secondcore layer and having a refractive index smaller than that of the secondcore layer; the multilayer structure is located below the secondcladding layer; and the thickness of the second cladding layer is set ata value equal to or more than 0.3 times and equal to or less than 2.1times of a distance where a beam intensity at the first waveguidedecreases from a maximum value to a value of exp(-2) times the maximumvalue in the first cladding layer.

In still another embodiment, the first waveguide includes a first corelayer which becomes a core for guiding the light, and a first claddinglayer located below the first core layer and having a refractive indexsmaller than that of the first core layer, the second waveguide includesa second core layer and a second cladding layer located below the secondcore layer and having a refractive index smaller than that of the secondcore layer, the multilayer structure is located below the secondcladding layer, and the thickness of the second cladding layer is avalue obtained by:

    mλ.sub.0 / 2n.sub.2 {1-(n.sub.0 sin θ/n.sub.2).sup.2 }.sup.1/2 !

where λ₀ is a wavelength of light in vacuum, n₀ is a refractive index ofthe first core layer, n₂ is a refractive index of the second claddinglayer, θ is an angle formed by a light propagating direction in thefirst core layer and a normal of the multilayer structure, and m is aninteger more than 0.

According to another aspect of the invention, a method for fabricatingan integrated optical control element is provided. The method includesthe steps of: forming a first waveguide layer structure including atleast a first layer which becomes a core for guiding light and secondand third layers located above and below the first layer, respectively,each of the second and third layers having refractive indexes smallerthan that of the first layer on the substrate; forming a first waveguideend face by etching part of the first waveguide layer structure; forminga multilayer structure on the first waveguide end face, the multilayerstructure including at least one layer having a refractive indexdifferent from an equivalent refractive index of a region with which theat least one layer is in contact; and forming a second waveguide layerstructure including a fourth layer which becomes a core for guiding thelight and is located at a position of the same vertical level as that ofthe fourth layer, and fifth and sixth layers located above and below thefourth layer, respectively, each of the fifth and sixth layers havingrefractive indexes smaller than that of the fourth layer.

Alternatively, the method for fabricating an integrated optical controlelement of this invention includes the steps of: forming a concavestructure on a substrate; and forming on the substrate consecutively afirst waveguide layer structure and a multilayer structure so as not tobe parallel to the first waveguide layer structure, the first waveguidelayer structure including a first layer as a core for guiding light andsecond and third layers located above and below the first layer,respectively, each of the second and third layers having refractiveindexes smaller than that of the first layer, the multilayer structurebeing located in the vicinity of a step of the concave structure, andincluding at least one layer having a refractive index different from anequivalent refractive index of a region with which the at least onelayer is in contact.

Alternatively, the method for fabricating an integrated optical controlelement of this invention includes the steps of: forming a step on asubstrate to form a first substrate region and a second substrate regionwith different heights; and forming on the substrate consecutively afirst waveguide layer structure, a multilayer structure which is locatednear the boundary of the first substrate region and the second substrateregion and is perpendicular to the first waveguide layer structure, anda second waveguide layer structure arranged to be parallel to the firstwaveguide layer structure, wherein the first waveguide layer structureincludes a first layer as a core for guiding light and second and thirdlayers located above and below the first layer, respectively, each ofthe second and third layers having a refractive index smaller than thatof the first layer; the multilayer structure includes at least one layerhaving a different refractive index from an equivalent refractive indexof a region with which the at least one layer is in contact; and thesecond waveguide layer structure includes a fourth layer which as a corefor guiding light and fifth and sixth layers located above and below thefourth layer, respectively, each of the fifth and sixth layers having arefractive index smaller than that of the fourth layer, and wherein thelevels of the vertical positions of the first layer at the firstsubstrate region and the fourth layer at the second substrate region areidentical to each other.

According to still another aspect of the invention, an opticalintegrated circuit element is provided, which includes a substrate and aplurality of optical elements formed integrally on the substrate, theoptical elements including at least one integrated optical controlelement described above.

In one embodiment, the optical integrated circuit element furtherincludes a multi-function semiconductor laser for supplying light to theat least one integrated optical control element, which outputs aplurality of coherent laser beams.

In another embodiment, the optical integrated circuit element is anoptical integrated circuit element for coherent detection and furthercomprises a laser for local oscillation for supplying light to the atleast one integrated light control element.

In still another embodiment, the at least one integrated optical controlelement is a light demultiplexing element for dividing one input beaminto a plurality of output beams.

In still another embodiment, the optical integrated circuit elementfurther comprises a plurality of waveguides, and the multilayerstructure is directly formed on a waveguide end face in the at least oneintegrated optical control element for guiding signal light input fromoutside.

Alternatively, the method for fabricating an integrated optical controlelement of this invention includes the steps of: forming a waveguideincluding at least a first layer as a core for guiding light, and secondand third layers located above and below the first layer, respectively,each of the second and third layers having refractive indexes smallerthan that of the first layer on a substrate; forming a concave portionin a portion of the waveguide, the concave portion extending through atleast the first layer and either one of the second and third layerswhich is closer to the substrate; and forming a multilayer structurecomposed of layers sequentially formed inside the concave portion, so asto bury the concave portion.

Alternatively, the integrated optical control element of this inventionincludes: a waveguide including a first layer as a core for guidinglight and second and third layers located above and below the firstlayer, respectively, each of the second and third layers havingrefractive indexes smaller than that of the first layer; a concaveportion formed in a portion of the waveguide, the concave portionextending through at least the first layer and either one of the secondand third layer which is closer to the substrate, the concave portiondividing the waveguide into a first waveguide portion and a secondwaveguide portion; and a multilayer structure formed inside the concaveportion by sequentially forming a plurality of layers inside the concaveportion.

In one embodiment, the refractive index of a layer formed finally amongthe plurality of layers constituting the multilayer structure is largerthan a refractive index obtained from n₀ sinθ where n₀ is a refractiveindex of the first layer and θ is an angle formed by a light propagatingdirection in the first layer and a normal of the multilayer structureand is smaller than that of the other layer or layers of the multilayerstructure.

In another embodiment, the refractive index of the thickest layer amongthe plurality of layers constituting the multilayer structure is largerthan a refractive index obtained from n₀ sinθ where n₀ is a refractiveindex of the first layer and θ is an angle formed by a light propagatingdirection in the first layer and a normal of the multilayer structureand smaller than a refractive index of the other layer or layers of themultilayer structure.

According to still another aspect of the invention, an opticalintegrated circuit device is provided, which includes a plurality ofoptical elements in combination, wherein at least one of the pluralityof optical elements is the integrated optical control element describedabove.

In one embodiment, the optical integrated circuit device furtherincludes a multi-function laser for supplying light to the integratedoptical control element, which outputs a plurality of coherent laserbeams.

In another embodiment, the optical integrated circuit device is anoptical integrated circuit device for coherent detection and furtherincludes a laser for local oscillation for supplying light to theintegrated optical control element.

In still another embodiment, the optical integrated circuit device is anoptical integrated circuit device for coherent detection where localoscillation light and signal light are input from outside to theintegrated optical control element including a plurality of waveguides,and the multilayer structure is directly formed on a waveguide end facein the integrated optical control element for guiding the signal light.

Thus, according to the present invention, in the integrated opticalcontrol element with the above structures, a multilayer structure isformed between a first waveguide and a second waveguide. The multilayerstructure includes one or more layers with refractive indexes differentfrom the equivalent refractive index of the surrounding region. Themultilayer structure reflects or transmits light input from a firstinput waveguide. The reflected light and the transmitted light arecoupled to second output waveguides. Thus, since the multilayerstructure which is a multilayer reflection film formed at end faces ofthe waveguides has a light coupling/dividing function, the size of theelement can be as small as the width of the waveguides.

In the above multilayer reflection film, the variation in reflectance(transmittance) is as small as ±1% for a large variation in wavelengthused of about ±30 nm and a large variation in operating temperature of±30° C. Thus, the element can operate stably even when the environmentis not stable.

By having the multilayer reflection film composed of a plurality oflayers each having a thickness expressed by (2m+1)λ₀ /(4ncosθ), where λ₀is a wavelength of light in vacuum, θ is an angle formed by a lightproceeding direction and a normal to a layer, n is a refractive index ofeach layer, and m is an integer more than 0, the respective layers cancontrol light efficiently.

The refractive index and the thickness of the multilayer reflection filmrequired for the control of the reflectance of the element iscontrollable with a precision of ±1% by the thin film formation processsuch as vapor phase crystal growth. Accordingly, the reflectance andtransmittance of the element can be precisely controlled, and highproduction yield and reproducibility can be secured.

The TM mode light is input to the multilayer reflection film, and thevertical end face of the first waveguide possibly forms an angle ofabout 45° with respect to the light proceeding direction. That is, theangle θ formed by the light propagating direction in the first layer andthe normal of the multilayer reflection film is 45°. Thus, the incidentlight is an S wave with respect to the end face, and therefore lightdividing/coupling can be realized by the two output waveguides crossingat right angles.

According to a method for fabricating an integrated optical controlelement of the present invention, after the formation of a firstwaveguide structure, a multilayer reflection film is formed on avertical end face of the first waveguide structure. Then, a secondwaveguide structure is formed in contact with the multilayer reflectionfilm so that core layers of the two waveguide structures are positionedat substantially the same vertical level.

According to an alternate method of the present invention, a verticalend face is formed first by forming a concave structure on a substrate.Then, first and second waveguides are formed simultaneously. Amultilayer reflection film is then formed on the vertical end face. Inan integrated optical control element fabricated in the above method,since the first and second waveguides are formed by the same process,they have the same structure. Thus, the refractive index and thicknessof each layer, as well as the level of the vertical position of a corelayer, are identical between the first and second waveguides.Accordingly, the coupling efficiency between the first input waveguideand the second output waveguide can be improved.

Thus, the invention described herein makes possible the advantages of(1) providing an integrated optical control element with a size ofseveral micrometers square where the variation in opticalcoupling/dividing ratio (reflectance and transmittance) in response to avariation in operating temperature and light wavelength used issufficiently small and the production yield and reproducibility of theelement is high due to a large allowance of the opticalcoupling/dividing ratio with respect to the process precision, (2) amethod for fabricating such an integrated optical control element, (3)an optical integrated circuit element using such an integrated opticalcontrol element, and (4) an optical integrated circuit device using suchan integrated optical control element.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C show the steps of the fabrication process of the elementof Example 1 according to the present invention.

FIGS. 2A to 2C show the steps of the fabrication process of the elementof Example 1 according to the present invention.

FIG. 3 is a view for describing light incidence, reflection, andtransmission at a multilayer reflection film in Example 1.

FIG. 4 shows the dependency of the reflectance of the multilayerreflection film upon the light incident angle in Example 1.

FIG. 5 shows the dependency of the reflectance of the multilayerreflection film upon the light wavelength in Example 1.

FIG. 6 shows the dependency of the reflectance of the multilayerreflection film upon the element temperature in Example 1.

FIG. 7 shows the dependency of the reflectance of the multilayerreflection film upon the variation in film thickness in Example 1.

FIG. 8 shows the dependency of the reflectance of the multilayerreflection film upon the variation in Al mole fraction in Example 1.

FIGS. 9A to 9C show the steps of the fabrication process of the elementof Example 2 according to the present invention.

FIGS. 10A to 10C show the steps of the fabrication process of theelement of Example 2 according to the present invention.

FIG. 11 shows the dependency of the reflectance of a multilayerreflection film upon the light incident angle in Example 2.

FIG. 12 is a top view of the element of Example 4.

FIGS. 13A and 13B show the steps of the fabrication process of theelement of Example 5 according to the present invention.

FIGS. 14A and 14B show the steps of the fabrication process of theelement of Example 5 according to the present invention.

FIG. 15 shows a configuration of the optical integrated circuit elementfor light multi-branching of Example 6.

FIG. 16 is a sectional view of the element of Example 7.

FIG. 17 is a top view of the element of Example 8.

FIG. 18 shows a waveguide structure of a conventionalmulti/demultiplexer having a Y-shaped branching structure.

FIG. 19 shows a waveguide structure of a conventional directionalcoupler.

FIG. 20A is a top view of a conventional multi/demultiplexer using atrench, and FIG. 20B is a sectional view thereof taken along the lineX-X' of FIG. 20A.

FIG. 21 shows a waveguide structure of a conventional star coupler.

FIGS. 22A and 22B show steps of the fabrication process of the elementof Example 9 according to the present invention.

FIG. 23A shows a step of the fabrication process of the element ofExample 9, and FIG. 23B is a view for describing the operation principleof the element of Example 9.

FIG. 24 shows the relationship between the deviation of the relativepositions of waveguides and the coupling efficiency between thewaveguides in Example 9.

FIGS. 25A and 25B show steps of the fabrication process of the elementof Example 10 according to the present invention.

FIG. 26A shows a step of the fabrication process of the element ofExample 10, and FIG. 26B is a view for describing the operationprinciple of the element of Example 10.

FIGS. 27A and 27B show steps of the fabrication process of the elementof Example 11 according to the present invention.

FIGS. 28A and 28B show the coupling efficiency and the reflectance ofthe element of Example 11.

FIGS. 29A to 29C show steps of the fabrication process of the element ofExample 12 according to the present invention.

FIGS. 30A to 30E show steps of the fabrication process of the element ofExample 12 according to the present invention.

FIG. 31 shows the reflectance characteristic of the element of Example12 when layers in a concave portion are formed so as to obtain areflectance of 50%.

FIG. 32 is a perspective view of the optical integrated circuit deviceof Example 13 according to the present invention.

FIG. 33 is a perspective view of the optical integrated circuit elementof Example 14 according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described by way of examples withreference to the accompanying drawings.

(EXAMPLE 1)

An integrated optical control element of Example 1 will be described.

First, referring to FIGS. 1A to 1C and 2A to 2C, a 2-beam laser deviceincluding the integrated optical control element of this example and amethod for fabricating the same will be described. FIGS. 1A to 1C and 2Ato 2C show steps of the fabricating process of the 2-beam laser deviceof this example.

As shown in FIG. 1A, an n-AlGaAs cladding layer 101 with a thickness of1.0 μm, an AlGaAs active layer 102 with a thickness of 0.08 μm, ap-AlGaAs first cladding layer 103 with a thickness of 0.2 μm, a p-AlGaAsguide layer 104 with a thickness of 0.2 μm, and a p-GaAs absorptionlayer 105 with a thickness of 0.06 μm are consecutively formed on a(100) plane n-GaAs substrate 100 by metal organic chemical vapordeposition (MOCVD).

Referring to FIG. 1B, a diffraction grating 120 having a pitch of about0.36 μm and a depth of about 0.18 μm is formed by normal 2-fluxinterference exposure and wet etching. This diffraction grating 120works as a third-order diffraction grating for laser light with awavelength of 780 nm. The third-order diffraction grating is a gratingwhich diffracts light so that the intensity of third-order diffractionlight components of the diffracted light is the highest. Also, in orderto suppress the light absorption loss of the laser element and obtain alarge coupling coefficient, the diffraction grating 120 is controlled tohave a section ratio of a convex portion to a concave portion thereof ofabout 1:5. The stripe direction of the diffraction grating 120 isaligned in the 010! direction.

As shown in FIG. 1C, a p-AlGaAs second cladding layer 106 with athickness of 0.6 μm and a p-GaAs contact layer 107 with a thickness of0.3 μm are formed on the resultant wafer with the diffraction grating120 by MOCVD.

Then, as shown in FIG. 2A, a vertical step 131 is formed on theresultant wafer by normal photolithography and reactive ion etchingusing a dielectric film 130 as a mask, so that a vertical face 132 is inthe (010) plane. That is, the vertical face 132 and the stripe directionof the diffraction grating 120 form 45°. The height of the vertical step131 is 3.0 μm, so that an etching bottom 133 reaches the GaAs substrate100.

As shown in FIG. 2B, using the dielectric film 130 as the mask, anAlGaAs multilayer reflection film 109, an AlGaAs lower cladding layer110 with a thickness of 0.6 μm, an AlGaAs core layer 111 with athickness of 0.8 μm, and an AlGaAs upper cladding layer 112 with athickness of 1.0 μm are selectively formed by MOCVD. The multilayerreflection film 109 is composed of three sets of an Al₀.8 Ga₀.2 As layerwith a thickness of 110 nm and an Al₀.1 Ga₀.9 As layer with a thicknessof 83 nm, i.e., total six layers. The substrate surface (etching bottom)133 formed by the etching is in the (100) plane, and the vertical face132 is in the (010) plane. Accordingly, these two faces arecrystallographically equivalent, meaning that a layer structure with asubstantially equal thickness can be formed on both faces. Further, thecrystal growth temperature is set at a temperature in the range of 700°C. to 750° C. where the dependency of the crystal growth rate upon theplane orientation is smaller. By this setting, even when the verticalface 132 is slightly displaced from the (010) plane due to a reasonregarding process control, a deviation of the thickness of each layer ofthe multilayer reflection film 109 formed on the vertical face 132 fromthe designed thickness can be minimized.

All of the layers 109 to 112 formed in the selective crystal growthprocess are not doped with impurities so that they have high resistance.An unnecessary polycrystalline film (not shown) deposited on thedielectric film 130 in the above process is removed together with thedielectric film 130 by etching after the selective crystal growthprocess.

The thus-fabricated wafer is composed of three sections, as shown inFIG. 2B: a distributed feedback (DFB) type laser section 134; ademultiplexer section 135 including the multilayer reflection film 109formed on the vertical face 132; and a passive waveguide section 136.

Subsequently, as shown in FIG. 2C, a stripe-shaped resistive electrode140 is formed on the top surface of the laser section 134, while aresistive electrode 141 is formed over the entire bottom surface of thesubstrate 100. The stripe-shaped resistive electrode 140 has a width of3 μm, and is arranged so that two strips in the <011> and <01-1>directions are perpendicular to each other near the multilayerreflection film 109. Then, mesa stripes 142, 143, and 144 with a widthof 3 μm and a height of 0.8 μm are formed on the thus-fabricated waferby normal photolithography and wet etching. Thus, ridge waveguides arefabricated. The mesa stripes (waveguides) 143 and 144 are arranged sothat they are on one line. The mesa stripe (waveguide) 142 and the mesastripes 143 and 144 are perpendicular to each other and arranged toshare the multilayer reflection film 109 formed on the vertical face132. The mesa stripes 142 and 144 correspond to the stripe-shapedelectrode 140 in the laser section 134.

Finally, the thus-fabricated wafer is cleaved into chips. Each face ofthe chips is coated with a SiN reflection protection film (not shown)for suppressing the reflection of laser light, and thus the 2-beam laserdevice is completed.

The operation principle of the 2-beam laser device with the abovestructure will be described.

The laser section 134 of the laser element constitutes a DFB laserhaving the third-order absorption-type diffraction grating 120 along thewaveguide 144, which emits laser light by applying a current between theelectrodes 140 and 141. In the DFB laser, unlike a normal Fabry-Perotlaser, laser oscillation can be generated, without an end face mirror,by the effect of distributed feedback of light by the diffractiongrating 120. Laser light generated by the DFB laser propagates throughthe waveguide 144 and is incident on the demultiplexer section 135including the multilayer reflection film 109.

As shown in FIG. 3, the multilayer reflection film 109 constituting thedemultiplexer section 135 is composed of three sets of the AlGaAs layerwith a thickness d2 of 110 nm and the AlGaAs layer with a thickness d1of 83 nm (total six layers). Incident light 20 input to the multilayerreflection film 109 from the DFB laser at an incident angle θ is dividedinto reflection light 21 reflected in a 2θ direction with respect to theincident direction and transmission light 22 passing through themultilayer reflection film 109 substantially straight.

FIG. 4 shows the dependency of the reflectance (and the transmittancewhich is 1--reflectance) of the multilayer reflection film 109 upon theincident angle θ. In FIG. 4, the characteristic obtained when light ofthe TM mode (the mode where the direction of the photoelectric field isvertical to the active layer 102) is incident from the DFB laser of thelaser section 134 is shown by dotted lines, while the characteristicobtained when light of the TE mode (the mode where the direction of thephotoelectric field is parallel to the active layer 102) is incident isshown by dash-dot lines.

In the element of Example 1, the incident angle θ is 45° and theoscillation mode of the DFB laser is the TM mode. Accordingly, thereflectance of 50% and the transmittance of 50% are obtained, and thusthe incident light 20 is substantially equally divided into thereflection light 21 and the transmittance light 22.

The allowable range of the angle θ formed between the vertical face 132and the direction of the mesa stripe 144 required to obtain theintensity ratio of the reflection light to the transmission light of 1:1is 42° to 47°, which is comparatively wide. Accordingly, high productionyield can be ensured even when an unintended change occurs in thefabrication process. Further, since the multilayer reflection film 109is formed by MOCVD, the refraction index of each layer of the film canbe controlled with a precision of 1% or less. Accordingly, theuniformity, controllability, and the reproducibility of the reflectanceand transmittance are high. The variations in reflectance andtransmittance can be as small as the order of 50±0.4%.

In the element of Example 1, the incident angle θ is 45°. Accordingly,the direction 2θ of the reflection light is vertical to the direction ofthe incident light. This means that the light from the waveguide 144 ofthe laser section 134 incident on and reflected from the multilayerreflection film 109 formed on the vertical face 132 enters the waveguide142 formed in the direction perpendicular to the incident direction.

On the other hand, the transmission light is input to the passivewaveguide section 136 composed of the 3-layer structure formed togetherwith the multilayer reflection film 109 (the lower cladding layer 110,the core layer 111, and the upper cladding layer 112) and the stripestructure (the mesa stripe 143). The core layer 111 of the passivewaveguide section 136 is formed at a vertical position of substantiallythe same level as that of the active layer 102 of the laser section 134where the peak of light exists. Thus, the light which has been emittedfrom the laser section 134 and passed through the demultiplexer section135 is coupled to the passive waveguide section 136. A high couplingefficiency between the laser section 134 and the passive waveguidesection 136 can be obtained by precisely controlling the depth of theetching at the formation of the vertical face 132 and the thickness ofeach layer at the selective crystal growth so as to allow the level ofthe vertical position of the center of the core layer 111 to be equal tothat of the center of the active layer 102. The precision of thisequality is required to be ±0.3 μm.

The same multilayer structure as the multilayer reflection film 109 ofthe demultiplexer section 135 formed on the vertical face 132 is formedon the substrate 100 in the fabrication process, which is located belowthe 3-layer structure in the passive waveguide section 136. In Example1, the core layer 111 and the lower cladding layer 110 are made as thickas 0.8 μm and 0.6 μm, respectively, so that light guided by the passivewaveguide 143 does not reach the multilayer reflection film 109 of thepassive waveguide section 136, and thus the multilayer reflection film109 does not adversely affect the waveguide mode of the waveguide 143.

Before reaching the passive waveguide 143, light which has propagatedthe waveguide 144 of the laser section 134 passes through thedemultiplexer section 135 which includes the multilayer reflection film109 and has no waveguide mechanism. This allows the light to diverge dueto a diffraction effect. However, since the length of the demultiplexersection 135 is only about 2 μm, the element characteristic is not sosignificantly affected by this section. Actually, it has been confirmedthat light of substantially the same amount as the light obtained whenthe two waveguides 143 and 144 are in direct contact, specifically,about 95% of the light obtained when the two waveguides are in directcontact, couples to the passive waveguide 143. Incidentally, in order toallow 80% or more of the light obtained when the two waveguides 143 and144 are in direct contact to couple to the passive waveguide 143, thelength of the demultiplexer section 135 is required to be 5 μm or less.

Thus, the two light beams separating at the multilayer reflection film109 in the demultiplexer section 135 are coupled to the waveguides 142and 143, propagate therethrough, and are output from the respective endfaces.

In addition to the output light in the above two directions, the laserlight is also output backward from the end face of the waveguide 144 ofthe laser section 134 due to the structure of the laser element. Thus,laser light is output in the total of three directions from one laserresonator. The light output backward is received by a light receivingelement disposed near the element and used for light amount monitoring.The remaining two output light beams are output outside the element,keeping the coherence between each other. The laser element is thus usedas a coherent 2-beam laser source.

FIG. 5 shows the dependency of the reflectance of the multilayerreflection film 109 having the above structure upon the laser lightwavelength. The laser wavelength used in the 2-beam laser device ofExample 1 is 850 nm, and the element is controlled so that both thereflectance and the transmittance are 50% at this wavelength. As isobserved from FIG. 5, even when there arise a variation in wavelength(generally about ±1 nm) due to an unintended change in the laser elementfabrication conditions and a variation in oscillation wavelength(generally about ±2 nm for a DFB laser) due to a variation intemperature of the laser element, the variation in reflectance of theelement is minor, which is in the range of 50%±0.3%. Accordingly, theelement of Example 1 does not require a control system for strictlykeeping the wavelength of the used laser light at a fixed value.

FIG. 6 shows the dependency of the reflectance of the multilayerreflection film 109 with the above structure upon the elementtemperature. As is observed from FIG. 6, even when the temperature ofthe element of Example 1 varies in the range of -5° C. to 70° C. whichis a temperature range generally allowable for home appliances, thereflectance of the element varies only in the range of 49% to 51%.Accordingly, the element of Example 1 does not require temperaturecontrol in the above temperature range. It does not need to monitor theoutput light power from a demultiplexer and conduct feedback control ofthe operation conditions of the demultiplexer based on the results ofthe monitoring by use of an external circuit, either.

Now, the controllability and the reproducibility in the fabricationprocess of the element of Example 1 will be described. FIG. 7 shows thevariation in reflectance when the thickness of the multilayer reflectionfilm 109 varies from the set value. As is observed from FIG. 7, in orderto control the reflectance at a precision of 50±1%, the controllabilityof the film thickness of about ±3% should be secured at the secondselective crystal growth by MOCVD in the above-described fabricationmethod. It is well known that the crystal growth by MOCVD allows thefilm thickness to be controlled at a precision of ±1% or less for boththe distribution in one wafer and the variation among wafers. Theexperimental results by the Inventors have confirmed that the filmthickness on the vertical face can also be controlled at the same levelof precision.

FIG. 8 shows a variation in reflectance when the Al mole fraction of thelayers of the multilayer reflection film varies from the above setvalue. As is observed from FIG. 8, it is understood that in order tocontrol the reflectance at a precision of 50±1%, the controllability ofthe Al mole fraction of about ±1% should be secured at the secondselective crystal growth by MOCVD in the above-described fabricationmethod. This condition can be fully satisfied by the controllability ofthe Al mole fraction obtained by the MOCVD method. Accordingly, thecontrollability and the reproducibility in the fabrication of themultilayer reflection film are satisfactorily high, and thus highproduction yield can be obtained.

The 2-beam laser device of Example 1 is applicable as the light sourcefor various laser interference measurement systems such as a laserspeedometer and a laser distance measuring device. In the conventionalsystems, one output light beam emitted from a conventional laser isdivided into two by an external demultiplexer. This makes theconfiguration of the resultant optical system complicated and large inmany cases. Also, the adjustment of the optical system is difficult.However, by using the element applying the present invention, theoptical structure can be made simple and small, and thus the adjustmentthereof can be made simple and easy.

(EXAMPLE 2)

A second example of the present invention will be described. In Example2, the integrated optical control element of the present invention isapplied to a coherent detection element.

Referring to FIGS. 9A to 9C and 10A to 10C, the structure and thefabrication method of the coherent detection element of Example 2 willbe described. FIGS. 9A to 9C and 10A to 10C show the steps of thefabrication process of the coherent detection element.

First, as shown in FIG. 9A, an n-InGaAs first etching stop layer 301with a thickness of 0.01 μm and an n-InP buffer layer 302 with athickness of 1.0 μm are formed on an n-InP substrate 300 by molecularbeam epitaxy using organic metal material (MOMBE).

Then, as shown in FIG. 9B, a diffraction grating 320 having a pitch of220 nm and a depth of 70 nm is formed on the buffer layer 302 by normalphotolithography using a 2-flux interference exposure method and wetetching.

As shown in FIG. 9C, an n-InGaAsP guide layer 303 with a thickness of0.25 μm, a quantum well active layer 304 under tensile strain active toan emitting wavelength of 1.55 μm composed of four InGaAs layers with athickness of 7 nm and three InGaAsP barrier layers with a thickness of13 nm, a p-InP first cladding layer 305 with a thickness of 0.3 μm, anInGaAs etching stop layer 306 with a thickness of 7 nm, a p-InP secondcladding layer 307 with a thickness of 0.7 μm, and a p-InGaAs contactlayer 308 with a thickness of 0.3 μm are consecutively formed on thediffraction grating 320 by MOMBE.

Then, a region of the above multilayer structure is etched to reach somelevel of the n-InP buffer layer 302 by normal photolithography andreactive ion beam etching. A patterned SiN film 330 is used as a mask.The etching conditions are controlled so that a vertical etching facecan be obtained. Then, as shown in FIG. 10A, the remaining portion ofthe n-InP buffer layer 302 is removed by a hydrochloric acid etchant toreach the first etching stop layer 301, and then the first etching stoplayer 301 is removed by a sulfuric acid etchant. By this etchingprocess, the etching face can be made vertical by the reactive ion beametching and simultaneously the selective etching can be obtained by wetetching. That is, the etching face vertical to the substrate 300 and asurface 331 of the substrate 300 are exposed. By adopting the selectiveetching, the etching depth can be equal to the total of the thicknessesof the layers 301 to 308. Further, as shown in FIG. 10A, a vertical face332 forming 45° with regard to the grooves of the diffraction grating320 and a vertical face 333 forming 90° with regard to the grooves ofthe diffraction grating 320 are formed simultaneously in the etchingprocess. The two vertical faces 332 and 333 thus form an angle of 45°.

As shown in FIG. 10B, using the SiN film 330 as the mask, a multilayerreflection film 309 with a total thickness of 0.5 μm, an InP lowercladding layer 310 with a thickness of 0.6 μm, an InGaAsP core layer 311with a thickness of 0.25 μm, an InP first upper cladding layer 312 witha thickness of 0.2 μm, an InGaAs third etching stop layer 313 with athickness of 7 nm, and an InP second upper cladding layer 314 with athickness of 1.0 μm are consecutively formed on the substrate having thevertical step by normal MOCVD. The multilayer reflection film 309 iscomposed of two sets of an InP layer (active to a wavelength of 1.45 μm)with a thickness of 173 nm and an InGaAsP layer (active to a wavelengthof 1.45 μm) with a thickness of 159 nm (total four layers). Thethicknesses of the InP layer and the InGaAsP layer on the vertical face332 are set to satisfy √2·λ₀ /(4·n) where λ₀ is the wavelength of lightpassing through the multilayer reflection film 309 in vacuum, and n isthe refractive index of each layer constituting the multilayerreflection film 309. After the above crystal growth process, the SiNfilm 330 and a polycrystalline layer formed thereon are removed byetching.

Thereafter, an ohmic electrode 340 with a width of 3 μm is formed on thep-contact layer 308 and an ohmic electrode 341 is formed over the entirebottom surface of the substrate 300. Then, as shown in FIG. 10C, mesastripes 350 to 353 crossing at right angles at the multilayer reflectionfilm 309 formed on the vertical face 332 are formed by normalphotolithography and selective etching. Specifically, first, using anorganic resist as a mask, unmasked portions are etched to a depth ofabout 0.9 μm from the wafer surface by ion etching. Then, only theremaining InP layers are removed by a hydrochloric acid etchant toexpose the InGaAs etching stop layers 306 and 313. By this selectiveetching, the thicknesses of the cladding layer 305 and the uppercladding layer 312 located above the waveguide layers (the guide layer303, the active layer 304, and the core layer 311) on the sides of themesa stripes 350 to 353 can be strictly controlled. Thus, the transversemode characteristic of the waveguide defined by the mesa stripes 350 to353 can be controlled with high precision.

Finally, the thus-fabricated wafer is cut into chips. The faces of eachchip are coated with a reflection prevention film (not shown) made of adielectric film, and thus the coherent detection element is completed.

Now, the operation of the coherent detection element of Example 2 havingthe above structure will be described.

The function of the element of Example 2 is to divide light from each ofthe input waveguides (mesa stripes) 350 and 353 into two beams, andsimultaneously couple each one beam from one of the input waveguideswith each one beam from the other input waveguide. The coupled beams arethen input to the output waveguides (mesa stripes) 351 and 352. Theinput waveguide 350 constitutes a distributed feedback laser (DFB-LD),which emits laser light (corresponding to local oscillation light) witha wavelength of 1.55 μm when a current flows between the electrodes 340and 341. Since the active layer 304 of the DFB-LD is composed of fourquantum well layers under tensile strain as described above, lightemission recombination between electrons in the conduction band andlight-weight holes represents a main carrier recombination process.Accordingly, TM mode light is provided with a larger optical gain thanTE mode light. Thus, it is possible to selectively oscillate TM modelight in the laser input waveguide 350.

The thus-generated local oscillation laser light having the TM modepropagates through the input waveguide 350 and incident on themultilayer reflection film 309 formed on the waveguide face (verticalface) 332 formed vertical to the substrate 300.

FIG. 11 shows the variation in light intensity reflectance when TM modelight is input to the multilayer reflection film 309 at an incidentangle of 0° to 60°. When light with a wavelength of 1.55 μm is input tothe multilayer reflection film 309 from the DFB laser at an incidentangle of 45° as in the case of Example 2, the multilayer reflection film309 works as a semi-transparent film with a reflectance of 10% and atransmittance of 90%.

The light passing through the multilayer reflection film 309 is theninput to the output waveguide 351 formed close to the multilayerreflection film 309 and propagates in the output waveguide 351. Lightreflected from the multilayer reflection film 309 proceeds in adirection vertical to the input waveguide 350 and is input to themultilayer reflection film 309 formed on the vertical face 333 verticalto the light proceeding direction. The incident angle at this time is 0°(parallel to the normal of the face). As is observed from FIG. 11, lightis little reflected when input at the incident angle of 0° (thereflectance is less than 0.1%). Accordingly, light is little influencedby the multilayer reflection film 309 on the vertical face 333, andpasses therethrough to be input to the output waveguide 352. Thus, thelight emitted from the input waveguide 350 is divided into two at themultilayer reflection film 309 on the vertical face 332 which forms a45° angle with respect to the light proceeding direction, and thedivided two beams are coupled to the output waveguides 351 and 352 eachof which constitutes a passive waveguide composed of the layer structureformed in the second selective crystal growth process.

On the other hand, signal light is input to the passive input waveguide353, and then input to the multilayer reflection film 309 formed on thevertical face 332 at an incident angle of 45°, as in the case of thelocal oscillation light. In this case, also, 10% of the incident lightis reflected to be input to the output waveguide 351, while 90% thereofpasses through the multilayer reflection film 309 so as to be coupled tothe output waveguide 352. Accordingly, the multilayer reflection film309 formed on the vertical face 332 divides each of the localoscillation light and the signal light into two and simultaneouslycouples these two different types of light. Thus, the coupled signallight and local oscillation light propagate in the output waveguides 351and 352 to reach the end faces thereof, and are then output from the endfaces. The two output beams are converted into electric signals byrespective high-speed light receiving elements (not shown) separatelydisposed, and resultant signals are demodulated by a signal processingcircuit. Thus, coherent detection can be realized.

Thus, by applying the present invention to the coherent detectionelement, the area occupied by the integrated optical control element(the multiplexer in this example) can be as small as about W² (where Wis the width of the waveguide). Then, the size of the integrated circuitelement for coherent detection can be made as small as 1 mm² or less.Further, as in Example 1, the variation in reflectance when theoperating temperature of the element varies in the range of 0° C. to 60°C. is as small as ±0.2%. This eliminates the necessity of controllingthe temperature of the element. Also, the variation in reflectance whenthe light wavelength varies in the range of 1.3 μm to 1.7 μm is as smallas ±0.4% or less. This makes it possible to use the element of the samestructure for a system using the wavelength within the above range. InExample 2, also, by using the incident angle dependency of thereflectance of the multilayer reflection film 309 shown in FIG. 11, theoutput waveguide 352 which includes the multilayer structure formed inthe second selective crystal growth process can be used as a passivewaveguide. In Example 1, one of the two divided light beams propagatesin the active waveguide. In Example 2, the waveguides 351 and 352 whichallow the coupled light to propagate therein can be passive waveguidesof the same structure. This reduces power consumption.

(EXAMPLE 3)

In Example 3, a modification of the element of Example 2 will bedescribed.

In general, in light coupling/dividing in coherent detection, it ispreferable to divide/couple the signal light and the local oscillationlight at the ratio of 1:1 in order to cancel in phase noise overlappingwith the signal light or the local oscillation light on a circuit. Inshort, the reflectance of the multilayer reflection film 309 ispreferably 50%. In order to achieve this, in Example 3, the element ofExample 2 is modified to have a multilayer reflection film consisting ofmore layers though the modified element is not shown. The multilayerreflection film of Example 3 is composed of five sets of the InGaAsPlayer and the InP layer (total ten layers) each of which has the samethickness as that of the multilayer reflection film 309. The reflectanceof 50% can be obtained by this structure. The entire thickness of themultilayer reflection film of this example increases to 1.66 μm due tothe increase in the number of layers. Thus, the etching depth at theformation of the vertical face should be increased accordingly. That is,the n-type cladding layer 302 constituting the laser should be thickenedaccordingly.

Thus, in Example 3, as in Example 2, a small-size integrated opticalcontrol element with high stability against the variations in operatingtemperature and wavelength used can be realized. Also, good productionyield and reproducibility of the element can be obtained.

(EXAMPLE 4)

In Example 4, another modification of Example 2 for obtaining the lightcoupling/dividing ratio of 1:1 (reflectance: 50%) will be described.

The basic structure of the modified element of Example 4 is the same asthat of the element of Example 2 shown in FIGS. 9A to 9C and 10A to 10C.The difference is that the incident angle at the multilayer reflectionfilm is larger. FIG. 12 is a top view showing the waveguides and themultilayer reflection film of the integrated optical control element ofExample 4.

Referring to FIG. 12, a multilayer reflection film 509 is composed oftwo sets of an InGaAsP layer with a thickness of 0.31 μm and an InPlayer with a thickness of 0.29 μm (total four layers). The element isarranged so that the normal of the multilayer reflection film 509 formsa 67° angle with respect to both local oscillation light output from awaveguide 550 corresponding to the DFB-LD and signal light input fromoutside the element and propagating in a passive waveguide 553. Thewaveguide 553 and a waveguide 552 are formed to be vertical to thecleaved surface of the element because the DFB-LD does not needwaveguide end face reflection and the input waveguide 553 shoulddesirably have an input end face substantially vertical to the waveguidein order to receive light from outside.

With the above arrangement, the reflectance of 47% can be obtained bythe multilayer reflection film 509 composed of only four layers, andthus the coupling/dividing ratio of 1:1 can be obtained.

Thus, by the element with a size of 1 mm², the local oscillation lightand the signal light propagating in the input waveguides 550 and 553,respectively, can be divided/coupled at the ratio of 1:1, and thecoupled light can be coupled to output the waveguides 551 and 552 so asto be output from the element. The element of Example 4 also has highstability against the variations in operating temperature and wavelengthused, and the operable ranges for the temperature and the wavelength aswide as those in Examples 1 and 2 can be secured.

In the element of this example, the reflectance varies largely with thevariation in the angle of the multilayer reflection film 509 withrespect to the waveguide direction. For example, the reflectance variesby ±3% or more with the variation in the angle by ±1°. Accordingly, theangle should be controlled with a precision of ±0.20° or less at theformation of the vertical face prior to the formation of the multilayerreflection film 509. Since the above precision directly relates to themasking precision in normal photolithography, it is easy to control theangle at a precision of ±0.20° or less. Thus, the angle of the verticalface (the incident angle), as well as the film thicknesses and the molefraction of each layer of the multilayer reflection film 509, can besatisfactorily controlled to realize the element of this example, andthereby high production yield and reproducibility are secured.

(EXAMPLE 5)

In Example 5, an element having substantially the same structure as thatof Example 1 is fabricated by a different fabrication method. FIGS. 13A,13B, 14A, and 14B show the steps of the fabrication process of theelement of this example.

First, as shown in FIG. 13A, an undoped Al₀.25 Ga₀.75 As lower claddinglayer 601 with a thickness of 0.7 μm, an Al₀.2 Ga₀.8 As core layer 602with a thickness of 0.3 μm, and an undoped Al₀.25 Ga₀.75 As uppercladding layer 603 with a thickness of 0.6 μm are formed on asemi-insulating GaAs substrate 600 having the (100) plane by molecularbeam epitaxy (MBE). An SiO₂ film with a thickness of 0.3 μm is formed onthe resultant wafer surface by sputtering, and then a right-angledequilateral triangle shaped portion of the SiO₂ film is etched by normalphotolithography and etching using a hydrofluoric acid etchant. Thelength of the two equal sides forming a right angle is 4 μm, and the twoequal sides extend in the <01-1> and <011> directions while theremaining side extends in the <001> direction.

Then, using the patterned SiO₂ film as a mask, a concave structure 610is formed by reactive ion beam etching using chloride gas so that thebottom of the structure reaches the GaAs substrate 600. The etchingdepth is about 2 μm. By optimizing the voltage at the etching at 200 to400 V, substantially vertical faces 611, 612, and 613 can be obtained.The angle formed by the vertical faces 611 and 612 and by the verticalfaces 611 and 613 is 45°.

As shown in FIG. 13B, using the SiO₂ film as a mask, a multilayerreflection film 620 composed of two sets of an AlAs layer and a GaAslayer (total four layers), and an Al₀.25 Ga₀.75 As burying layer 621with a thickness of 1.5 μm are consecutively formed by MOCVD selectivegrowth. The multilayer reflection film 620 has a thickness whichsatisfies expression (1) below at the portion thereof formed on thevertical face 611 which forms a 45° angle with respect to the lightproceeding direction.

    (2m+1)λ.sub.0 /(4n cos θ)                     (1)

where λ₀ is the wavelength of light in a vacuum (1.3 μm), θ is the angleformed by the light proceeding direction and the normal of the layer(45°), n is the refractive index of the layer for 1.3 μm wavelengthlight, and m is an integer more than 0.

In this growth process, crystal deposition on the SiO₂ film can becompletely prevented by mixing a minute amount of hydrochloric gas in agas used for the crystal growth. Also, in this selective crystal growth,since most of the wafer surface is covered with the SiO₂ film,preventing the crystal to be deposited thereon, the growth species areconcentrated in the concave structure 610. This increases the growthrate of the low Al mole fraction (in this case, the GaAs layer and theAl₀.25 Ga₀.75 As layer) by about five times, compared with the normalcrystal growth on a flat substrate. In the actual crystal growth, theconditions should be set in consideration of this effect.

FIG. 14A is a sectional view of the wafer shown in FIG. 13B taken alongthe line A-A' of FIG. 13B. By the MOCVD selective crystal growth, theconcave structure 610 is completely buried, so that the surface of theresultant wafer is substantially flat.

Thereafter, as shown in FIG. 14B, a T-shaped mesa stripe 630 extendingin the <01-1> and <011> directions which are perpendicular to each otheris formed by normal photolithography and wet etching. The mesa stripe630 has a width of 3 μm and a height of 0.4 μm. The mesa stripe 630 isformed so that the crossing of the two bars of the T shape correspondsto the position of the multilayer reflection film 620 formed on thevertical face 611. Ridge waveguides 631, 632, and 633 are formed fromthe mesa stripe 630.

Finally, the resultant wafer is cleaved into chips, and three waveguideend faces of each chip are coated with anti-reflection coating film madeof Si₃ N₄ (not shown) by the plasma chemical vapor deposition (P-CVD)method.

Thus, an integrated optical control element for light branching isfabricated. In this element, light output from a fiber and/or a laserdiode is input to the input waveguide 631 at the end face thereof, anddivided into two at the multilayer reflection film 620, so as to becoupled to the output waveguides 632 and 633. In general, the lightoutput from the element is input to a fiber again. In Example 5, lightwhich has propagated through a fiber with a wavelength of 1.3 μm iscoupled to the input waveguide 631 and then divided equally. The dividedtwo beams are output from the element and coupled to two fibers.

In the element of Example 5, the size of the concave structure 610including the multilayer reflection film 620 having the light branchingfunction is 5 μm² or less. Thus, the entire element can be as small as250 μm². A light branching ratio of substantially 1:1 can be obtained.The variation in reflectance or transmittance is only about ±2% for thevariation in wavelength of ±0.1 μm and the variation in operatingtemperature of ±40° C. Thus, very stable characteristics can beobtained.

Unlike the elements of Examples 1 to 4, the input waveguide 631 and thetwo output waveguides 632 and 633 have the same layer structure formedin the same crystal growth process. Accordingly, the refractive indexand the thickness of each layer and the level of the vertical positionof the core layer 602 of the input waveguide 631 are identical to thoseof the output waveguides 632 and 633. This makes it possible to have acoupling efficiency as high as 95% between the input waveguide 631 andthe output waveguides 632 and 633. At this time, the expanse of lightdue to light diffraction at the Al₀.25 Ga₀.75 As burying layer 621 whereno waveguide structure exists can be negligibly small by reducing thelength of this portion to 3 μm or less. As a result, the optical loss ofthe entire element including the waveguide loss (which becomes smalleras the size of the element is smaller) can be 1.5 dB or less.

(EXAMPLE 6)

In Example 6, an optical integrated circuit element for multi-branchingapplying the element of Example 5 will be described. The element of thisexample divides one input light signal into eight equal light signals.

FIG. 15 is a top view showing the configuration of the opticalintegrated circuit element of Example 6. Each triangle mark in FIG. 15shows the concave structure 610 including the multilayer reflection film620 of the integrated optical control element described in Example 5.Light input to the end face of an input waveguide 700 from a fiber 712via a lens 711 is divided into two in power as the light passes througheach of integrated optical control elements 710. As described above, theintegrated optical control element 710 divides the power of input lightinto two equally at the multilayer reflection film thereof. The twodivided beams respectively pass through three integrated optical controlelements, and finally eight light signals having the same power areoutput to eight output waveguides 701 to 708. The eight light signalsare then output to fibers through lenses.

As described in Example 5, the size of the integrated optical controlelement 710 is about 5 μm². Accordingly, the size of the entire elementis restricted by the position of the outlets of the device connecting tofibers. In Example 6, since each two outputs are allocated on the fourfaces, the size of the entire package can be as small as 30 mm². Sincethe branching characteristic (reflectance and transmittance) withrespect to the variations in operating temperature and wavelength isstable, the operation of the optical integrated circuit element itselfis stable against the variations in temperature and wavelength. Theelement of Example 6 can be applied as an element (passivedemultiplexer) for an optical communication system in the case wherecommunication is conducted by use of one fiber between a plurality ofusers by the time-division multiplex communication system and in thecase where the same signal is distributed to a plurality of users as incable broadcasting.

(EXAMPLE 7)

The fabrication methods described in the above examples required twothin film formation processes. In Example 7, an integrated opticalcontrol element is fabricated by one thin film formation process. FIG.16 is a sectional view of the element of Example 7. Referring to FIG.16, the fabrication method of the element will be described.

First, a concave portion 810 with a right-angled equilateral triangleshape as is viewed from top is formed on a glass substrate 800 by normalphotolithography and dry etching. The shape of the concave portion 810viewed from top is the same as that of Example 5. The concave portion810 has a vertical face 811 which is to form 45° with respect to thelight proceeding direction (corresponding to the oblique side of theright-angled equilateral triangle) and vertical faces 812 and 813(corresponding to two sides perpendicular to each other). In thisexample, the length of the two sides perpendicular to each other is 4μm, and the depth of the concave portion 810 is 2.8 μm. Then, an SiO₂buffer layer 801 with a thickness of 0.3 μm, an Si₃ N₄ core layer 802with a thickness of 0.2 μm, an SiO₂ cladding layer 803 with a thicknessof 0.5 μm, and a multilayer reflection film 804 are consecutively formedon the substrate 800 by P-CVD. The multilayer reflection film 804 iscomposed of three Si₃ N₄ layers and three SiO₂ layers and has athickness satisfying expression (2) below, as in Example 5.

    (2m+1)λ.sub.0 /(4n cos θ)                     (2)

where λ₀ is the wavelength of light in a vacuum (800 nm), θ is the angleformed by the light proceeding direction and the normal of the layer(45°), n is the refractive index of the layer for 800 nm wavelengthlight, and m is an integer more than 0.

In the P-CVD method, thin films can be formed on a flat surface and on avertical surface at substantially the same growth rate. Thus, themultilayer reflection film 804 with the thickness satisfying equation(2) can be formed on the vertical faces 811 and 812.

Then, an SiO₂ burying layer 805 is formed on the entire surface of thewafer by spin coating so as to have a thickness of 0.3 μm at the flatportion. By employing the spin coating method for the formation of theSiO₂ burying layer 805, it is possible to fill the concave portion 810after the P-CVD process to completely flatten the wafer surface.

Thereafter, as in Example 5, mesa stripes, i.e., waveguides are formedby normal photolithography and ion etching. FIG. 16 is a sectional viewof the portion of the element where the mesa stripe is formed, takenalong the length of the mesa stripe. The waveguides form a cross shapecrossing at the portion where the multilayer reflection film 804 isformed.

Finally, the wafer is cut by a diamond blade to form the element. Thecut faces should be substantially perpendicular to the waveguides sothat the aligning with fibers at light inputs/outputs can be easy.

Light with a wavelength of 800 nm is input to an input waveguide 820 ofthe thus-fabricated element. The light propagating through the inputwaveguide 820 is input to the multilayer reflection film 804 formed onthe vertical face 811 at an incident angle of 45°. Part of the light isreflected from the multilayer reflection film 804. The remainder of thelight which has passed through the multilayer reflection film 804 thenpasses through the SiO₂ burying layer 805 and reaches the multilayerreflection film 804 formed on the vertical face 812. As described inExample 2, the reflectance of light input to the multilayer reflectionfilm 804 perpendicularly is less than 0.1%. Accordingly, light is littlereflected from the multilayer reflection film 804 formed on the verticalface 812, and most of the light is coupled to an output waveguide 821.The light reflected from the multilayer reflection film 804 formed onthe vertical face 811 forming 45° with respect to the light proceedingdirection proceeds in a direction perpendicular to the plane of FIG. 16,and is coupled to an output waveguide not shown.

As in the previous examples, the basic structure used for the lightbranching/coupling is the multilayer reflection film composed ofdielectrics, which can be formed in an area as small as 3 μm². Thus, theentire element can be as small as 200 μm². The branching/coupling ratiocan be kept at 1:1 with a precision of ±1% for the variation inoperating temperature of ±30° C. and the variation in wavelength of ±50nm.

Since the light input and the two light outputs of the element areformed on different faces, the entire size of the element including apackage can be made small.

As in Example 5, the input waveguide 820, the output waveguide 821, andan output waveguide not shown which is located perpendicular to theplane of FIG. 16 have the same waveguide structure formed in the samethin film formation process and the vertical positions thereof are atthe same level. Thus, the light coupling efficiency between the inputwaveguide and the output waveguides can be as high as 95%. This makes itpossible to suppress the light loss at the integrated optical controlelement of this example to 1.2 dB or less.

Moreover, by previously forming the deep concave portion 810 on thesubstrate 800, the light branching/coupling element composed of thepassive waveguides can be fabricated by one thin film formation process.Accordingly, the element fabrication process can be simplified, theproduction yield of the element can be further improved, and theproduction yield of the optical integrated circuit element using theelement of this example can be improved.

(EXAMPLE 8)

In Example 8, an element integrating a laser element and a waveguidetype light receiving element will be described.

FIG. 17 is a plan view of the element of Example 8. The basic structureof the element of this example is the same as that of Example 2.Accordingly, in FIG. 17, components similar to those in Example 2 aredenoted by the same reference numerals as those in FIGS. 9A to 9C and10A to 10C.

The element of Example 8 is different from the element of Example 2 inthe structure of layers 309 to 314 corresponding to the waveguidestructure formed at the second crystal growth. In Example 8, thestructure is composed of an iron-doped semi-insulating multilayerreflection film 309, a p-InP lower cladding layer 310, a p-InGaAs corelayer 311 having a composition absorbing laser light, an n-type InPfirst upper cladding layer 312, an n-type InGaAs third etching stoplayer 313, and an n-InP second upper cladding layer 314. The thicknessof each layer is the same as that in Example 2.

N-type ohmic electrodes 951 and 952 are formed on output waveguides(mesa stripes) 351 and 352. A zinc diffusion region reaching the p-typelower cladding layer 310 is formed on a portion of the wafer where themesa stripes are not formed, and a p-type ohmic electrode 953 is formedon the zinc diffusion region.

The operation principle of the thus-fabricated element will bedescribed. Light of the TM mode is input to the multilayer reflectionfilm 309 from an input waveguide 350 constituting a DFB laser at anincident angle of 45°. Similarly, signal light introduced to an inputwaveguide 353 from outside is input to the multilayer reflection film309 at an incident angle of 45°. The length of the input waveguide 353in the light propagating direction is preferably as small as possiblebecause the input waveguide 353 constitutes a light receiving elementand thus has large light absorption. In this example, it is 40 μm. Thisvalue is determined in consideration of the reliability of the inputwaveguide (laser portion) 350. The light incident on the multilayerreflection film 309 is branched/coupled according to the reflectanceshown in FIG. 11 and introduced to the output waveguides 351 and 352which form a wavelength type light receiving element. The outputwaveguides 351 and 352 are electrically isolated from the laser portion350 by the semi-insulating multilayer reflection film 309, and a reversebias voltage can be applied between the electrodes 951 and 953 andbetween the electrodes 952 and 953. This makes it possible to realize alight receiving element with high-speed response. In the element ofExample 8, a signal with a beat frequency of 5 GHz corresponding to thefrequency difference between the signal light and the local oscillationlight can be detected.

As described above, in the element of Example 8, the output waveguides351 and 352 are used as a light receiving element capable of beingindependently driven. Accordingly, in addition to the effect describedin Example 2, the coherent detection function can be performed only bythis element. This eliminates the necessity of disposing a lightreceiving element outside the element by aligning it with the element asin Example 2. Thus, the coherent detection system can be simplified andmade small.

(EXAMPLE 9)

In Example 9, an integrated optical control element where both awaveguide structure and an optical control structure (layer) are formedby one thin film formation process will be described.

First, as shown in FIG. 22A, a vertical step 2210 is formed on a (001)GaAs substrate 2200 by photolithography and dry etching. The directionof the step 2210 is set at the 010! direction so that the vertical faceof the vertical step 2210 is substantially in the plane (010). Theheight of the step 2210 is set at 2.0 μm by controlling the etchingtime.

Then, as shown in FIG. 22B, an Al₀.25 Ga₀.75 As first lower claddinglayer 2201 with a thickness of 0.8 μm, an Al₀.2 Ga₀.8 As first corelayer 2202 with a thickness of 0.2 μm, an Al₀.25 Ga₀.75 As first uppercladding layer 2203 with a thickness of 0.8 μm, an Al₀.8 Ga₀.2 Asoptical control layer 2204 with a thickness of 0.26 μm, an Al₀.25 Ga₀.75As second lower cladding layer 2205 with a thickness of 0.8 μm, an Al₀.2Ga₀.8 As second core layer 2206 with a thickness of 0.2 μm, and anAl₀.25 Ga₀.75 As second upper cladding layer 2207 with a thickness of0.8 μm are consecutively formed on the substrate 2200 having the step2210 by a normal-pressure MOCVD crystal growth method, for example.

At this crystal growth, in order to form the layers 2201 to 2207substantially keeping the shape of the substrate 2200 as shown in FIG.22B, the conditions are set as follows: The substrate temperature is aslow as 600° C. to 680° C.; the mole ratio of the III-group atoms to theV-group atoms in the vapor is as large as 130 to 250; and the migrationdistance of adsorbed atoms or molecules on the crystal surface is short.

Then, as shown in FIG. 23A, a T-shaped mesa stripe 2211 with a width of3.0 μm and a height of 4.0 μm is formed by normal photolithography anddry etching using an Si₃ N₄ film (not shown) as a mask. At this time,the mesa stripe 2211 is formed so that the crossing of two bars of the Tshape corresponds to the position of the step 2210.

Subsequently, using the Si₃ N₄ film used at the formation of theT-shaped mesa stripe 2211 as the mask, an Al₀.25 Ga₀.75 As burying layer2208 with a thickness of 4.0 μm is selectively formed so as to bury theportions other than the T-shaped mesa stripe 2211.

Finally, after the removal of the Si₃ N₄ film used as the mask for theselective crystal growth, the wafer is cleaved to form an element havingfour faces including three faces corresponding to the light input/outputfaces 2220 to 2222 as shown in FIG. 23A.

The thus-fabricated element includes a first waveguide 2212 composed ofthe first lower cladding layer 2201, the first core layer 2202, and thefirst upper cladding layer 2203 and a second waveguide 2213 composed ofthe second lower cladding layer 2205, the second core layer 2206, andthe second upper cladding layer 2207. The second waveguide 2213 is puton the first waveguide 2212 in the layer thickness direction via anoptical control layer 2204.

Now, the operation principle of the element of Example 9 will bedescribed with reference to FIGS. 23A and 23B. FIG. 23B is a partialsectional view of the element taken along the line B-B' of FIG. 22Bshowing the portion at and around the vertical step 2210.

First, signal light with a wavelength of 780 nm is input to the secondwaveguide 2213 composed of the second lower cladding layer 2205, thesecond core layer 2206, and the second upper cladding layer 2207, andpropagates in the second waveguide 2213. The light propagating in thesecond waveguide 2213 then reaches the vertical step 2210. Due to thevertical bend of the second core layer 2206, the light enters the secondlower cladding layer 2205 and then enters the optical control layer 2204interposed between the second lower cladding layer 2205 and the firstupper cladding layer 2203. Since the optical control layer 2204 has astep portion substantially corresponding to the vertical step 2210, thelight is input to the optical control layer 2204 at an incident angle ofabout 45°. The Al₀.8 Ga₀.2 As optical control layer 2204 having athickness of 0.26 μm satisfies the following expression (3) for thelight with a wavelength of 780 nm incident thereon at an angle of about45°.

    3λ.sub.0 /(4n cos 45°)                       (3)

where λ₀ =780 nm and n=3.12. Due to this multi-reflection effect in theoptical control layer 2204, 10% in intensity of the light incident onthe optical control layer 2204 is reflected, while the remaining 90% inintensity of the light passes through the optical control layer 2204.

The light reflected from the optical control layer 2204 proceeds in adirection turned by 90° (direction vertical to the plane of FIG. 23B).It propagates in the second waveguide 2213 again and is output outsidethe element from a sub-output end face 2221. The light which has passedthrough the optical control layer 2204 is coupled to the first waveguide2212 including the first core layer 2202 as the center. After havingpropagated through the first waveguide 2212, the light is output outsidethe element from a main output end face 2222.

In the element of Example 9, the height of the step 2210 (2.0 μm) andthe total thickness of the first core layer 2202, the first uppercladding layer 2203, the optical control layer 2204, and the secondlower cladding layer 2205 are controlled under the conditions that thestructures of the first waveguide 2212 and the second waveguide 2213 arethe same and that the vertical positions of the first core layer 2202and the second core layer 2206 are at the same level with a precision ofthe thickness of the core layer 2202 or 2206 or less. Thus, as shown inFIG. 24, the light radiation loss at the optical coupling from thesecond waveguide 2213 to the first waveguide 2212 can be as small asabout 3.0 dB or less.

More preferably, by controlling under the condition that the verticalpositions of the first core layer 2202 and the second core layer 2206are at the same level with a precision of about a half of the thicknessof the core layer 2202 or 2206, the light radiation loss at the opticalcoupling from the second waveguide 2213 to the first waveguide 2212 canbe further as small as 1.0 dB or less.

As in the previous examples, the light multi-reflection effect in theoptical control layer 2204 is utilized for the reflectance andtransmittance. Accordingly, the variation in reflectance andtransmittance with the variation in temperature in the range of 0° to60° C. and the variation in wavelength in the range of 770 to 790 nm canbe as small as ±1%. Thus, the variation in intensity of light outputfrom the sub-output end face 2221 can be stabilized within ±1%. Further,the variation in reflectance and transmittance is as small as about ±1%for the variation in layer thickness and mole fraction by ±5% in theMOCVD crystal growth process. This allows for a sufficiently largeallowance for the fabrication process. As a result, high reproducibilityat the fabrication of the element and a production yield as high as 99%or more were obtained.

As is easily understood from the structure of the element of Example 9,the major portion of the element is the optical control layer 2204, thefirst waveguide 2212, and the second waveguide 2213 located at andaround the vertical step 2210. The size of this region is determined bythe width of the waveguides. In this case, it is as small as 3.0 μm².

Thus, a very small integrated optical control element with highcharacteristic stability and controllability at fabrication can beobtained, which, in addition to main output, provides a sub-output usedfor sampling for the purpose of monitoring the intensity, thewavelength, and the like.

(EXAMPLE 10)

In Example 10, an integrated optical control element for coherentdetection will be described with reference to FIGS. 25A, 25B, 26A, and26B.

First, a waveguide portion of the element is fabricated as follows. Asshown in FIG. 25A, an n-Al₀.5 Ga₀.5 As first cladding layer 2301 with athickness of 1.9 μm, an Al₀.3 Ga₀.7 As core layer 2302 with a thicknessof 0.3 μm, an Al₀.5 Ga₀.5 As second cladding layer 2303 with a thicknessof 0.45 μm, a GaAs etching stop layer 2304 with a thickness of 3 nm, andan Al₀.5 Ga₀.5 As third cladding layer 2305 with a thickness of 0.45 μmare consecutively formed on an n-GaAs substrate 2300 by MOCVD, forexample.

Then, a patterned dielectric film 2306 is formed on the resultant wafer.Using the dielectric film 2306 as a mask, the above layers areselectively etched by normal reactive ion dry etching, leaving part ofthe first cladding layer 2301 as a thin layer. A V-shaped face 2307formed by dry etching stand at 90°±1° with respect to the surface of thecore layer 2302. The etching depth is 2.6 μm with a precision of ±0.1μm.

Then, a laser section for local oscillation light is fabricated asfollows. As shown in FIG. 25B, a semiconductor multilayer reflectionfilm 2308 is formed on the first cladding layer 2301 exposed by theabove dry etching by the MOCVD crystal growth technique. The multilayerreflection film 2308 is composed of three sets of an n-Al₀.18 Ga₀.82 Aslayer with a thickness of 84 nm and an n-Al₀.82 Ga₀.8 As layer with athickness of 112 nm (total six layers). On the multilayer reflectionfilm 2308 are consecutively formed an n-AlGaAs fourth cladding layer2309 with a thickness of 0.9 μm, an AlGaAs active layer 2310 with athickness of 0.1 μm, a p-AlGaAs optical guide layer 2311 with athickness of 0.20 μm, and a p-GaAs optical absorption layer 2312 with athickness of 0.06 μm.

Thereafter, a diffraction grating is formed using the optical absorptionlayer 2312 and the optical guide layer 2311 by normal electron beamdrawing and wet etching. The diffraction grating has a pitch of 0.13 μmand a depth of 0.12 μm, and functions as a first-order diffractiongrating for 0.85 μm-wavelength light. In order to obtain the maximumdiffraction effect, the area ratio of the optical guide layer 2311 tothe optical absorption layer 2312 at the section of the diffractiongrating is set at 1:1.

Then, a p-AlGaAs fifth cladding layer 2313 with a thickness of 0.29 μm,a p-AlGaAs etching stop layer 2314 with a thickness of 3 nm, and ap-AlGaAs sixth cladding layer 2315 with a thickness of 0.45 μm areconsecutively formed on the diffraction grating. A semiconductor filmformed on the dielectric film 2306 is then removed by etching.

As shown in FIG. 26A, the third cladding layer 2305 and the sixthcladding layer 2315 are etched by normal photolithography and wetetching so as to form mesa-shaped waveguides 2320 to 2323. At this time,the mesa portions should form an angle of 45° with respect to the largerportion of the V-shaped face 2307. Finally, an electrode 2316 is formedon the mesa portion of the laser section.

Next, the operation of the integrated optical control element forcoherent detection will be described. A light signal is introduced tothe waveguide 2320 from an end face 2319. The light signal is dividedinto two at a ratio of about 1:1 at the multilayer reflection film 2308and input to the waveguides 2321 and 2322.

Local oscillation light emitted from the laser section propagatesthrough the waveguide 2323 and input to the waveguides 2321 and 2322 viathe multilayer reflection film 2308. There exists a region in thewaveguides at and around the multilayer reflection film 2308 where thecore layer 2302 and the active layer 2310 for conducting lightconfinement in the vertical direction are not formed. This region isshown in FIG. 26B by shading. The light propagating in the waveguides islargely diffracted in the vertical direction at this region, degradingthe coupling efficiency between the waveguides. In Example 1, thiseffect is hardly observed because the coupling is conducted from the0.08 μm thick core to the 0.8 μm thick core. In Example 10, however,since the waveguides 2320, 2321, and 2322 have the same core thickness,this influence is not negligible. Actually, the coupling loss from thewaveguide 2320 to the waveguide 2321 can be suppressed to about 5%because only the multilayer reflection film 2308 exists between thewaveguides. On the contrary, the coupling loss from the waveguide 2320to the waveguide 2322 is as large as about 1 dB because the fourthcladding layer 2309 with a thickness of 0.9 μm exists between thewaveguides and the light largely diffracts during passing the longdistance of the shaded portion as shown in FIG. 26B.

When the local oscillation light is coupled to the waveguides 2321 and2322, the coupling loss to the waveguide 2321 is about 1 dB, while thatto the waveguide 2322 is about 2 dB.

In general, in the coherent detection, signal light received fromoutside tends to be weak compared with the local oscillation light. Thesignal-to-noise ratio (S/N) is determined by the receiving level of thesignal light. Accordingly, a coherent detection element with highersensitivity can be obtained by having no light-diffracting layer betweenthe end face for receiving signal light and light receiving elements(disposed at the light output end faces of the waveguides 2321 and 2322though not shown).

Incidentally, the above effect can also be obtained by the elements ofthe above examples, when the input waveguide for signal light and thewaveguide connected to a light receiving element are connected without astructure interposed therebetween except for the multilayer reflectionfilm as in this example.

The laser section for local oscillation may not necessarily be formedintegrally as in Example 10. Instead, a separate DFB laser and the likemay be connected to the waveguide 2323 for coherent detection. The sameeffect as described above will be obtained by the this structure.

(EXAMPLE 11)

In Example 11, a 2×2 optical branching waveguide (an integrated opticalcontrol element) will be described.

Referring to FIGS. 27A and 27B, the fabrication method for the opticalbranching waveguide will be described.

As shown in FIG. 27A, an n-Al₀.5 Ga₀.5 As first cladding layer 2401 witha thickness of 1.29 μm, an Al₀.3 Ga₀.7 As core layer 2402 with athickness of 0.3 μm, an Al₀.5 Ga₀.5 As second cladding layer 2403 with athickness of 0.45 μm, a GaAs etching stop layer 2404 with a thickness of3 nm, and an Al₀.5 Ga₀.5 As third cladding layer 2405 with a thicknessof 0.45 μm are consecutively formed on a GaAs substrate 2400 by MOCVDcrystal growth, for example.

Then, using a dielectric film (not shown) as a mask, the above layersare selectively etched by a depth of 2.0 μm by normal reactive ion dryetching, leaving part of the first cladding layer 2401 as a thinnerlayer.

Further, a semiconductor multilayer reflection film 2406 composed ofthree sets of an Al₀.18 Ga₀.82 As layer with a thickness of 84 nm and anAl₀.82 Ga₀.18 As layer with a thickness of 112 nm in this order (totalsix layers) is formed.

On the multilayer reflection film 2406 are consecutively formed an Al₀.5Ga₀.5 As fourth cladding layer 2407 with a thickness of 0.19 μm, anAl₀.3 Ga₀.7 As core layer 2408 with a thickness of 0.3 μm, a Al₀.5 Ga₀.5As fifth cladding layer 2409 with a thickness of 0.55 μm, a GaAs etchingstop layer 2410 with a thickness of 3 nm, and an Al₀.5 Ga₀.5 As sixthcladding layer 2411 with a thickness of 0.45 μm by MOCVD crystal growth,for example. The order of the formation of the Al₀.18 Ga₀.82 As layerand the Al₀.82 Ga₀.18 As layer of the multilayer reflection film 2406does not influence the reflection characteristic of the film. However,in this example, for the reason described below, the Al₀.18 Ga₀.82 Aslayer is formed before the Al₀.82 Ga₀.18 As layer, and these layers aresequentially stacked. That is, the layer with the higher refractiveindex is formed before the layer with lower refractive index.

After removing a semiconductor layer formed on the dielectric film bywet etching, mesa-shaped waveguides 2412 to 2415 are formed using thepatterned dielectric mask as shown in FIG. 27B. As in Example 10, thewaveguides are formed so as to have an angle of 45° against themultilayer reflection film 2406. The multilayer reflection film 2406 hasa reflectance of 50% for 0.85 μm-wavelength light incident thereon at45°.

The characteristics of the optical branching waveguide of Example 11will be described.

The coupling efficiency of the waveguide 2413 to the waveguide 2412 isdegraded when the fourth cladding layer 2407 is thick and the gapbetween the waveguides is large. However, when the fourth cladding layer2407 is thin, inconsistency in mode arises between the waveguides 2412and 2413 due to influence from the semiconductor multilayer reflectionfilm, degrading the coupling efficiency therebetween. The Inventors haveset the thickness (T) of the fourth cladding layer to satisfy:

    0.3×MW<T<2.1×MW                                (4)

where MW is the mode expanse width of the 0-order mode of the waveguide2412 in a downward direction, which is the distance where the lightpower at the waveguide 2412 decreases from the peak value to a value ofexp(-2) times the peak value.

FIG. 28A shows the coupling efficiency between the waveguides 2412 and2413 when the thickness of the fourth cladding layer/MW is varied. As isobserved from FIG. 28A, high coupling efficiency is obtained when theabove condition, 0.3×MW<T<2.1×MW is satisfied.

In order to reduce the influence of the semiconductor multilayerreflection film to the mode, the Inventors decided that the layer with alower refractive index of the multilayer reflection film 2406 should bein contact with the fourth cladding layer. If the layer with a higherrefractive index of the multilayer reflection film 2406 is in contactwith the fourth cladding layer, a coupling loss of about 5% to 15%arises.

On the other hand, the reflectance may vary depending on the thicknessof the fourth cladding layer 2407. This problem can be overcome bysetting the thickness of the fourth cladding layer 2407 at or around avalue obtained by expression (5):

    mλ.sub.0 / 2n.sub.2 {1-(n.sub.0 sin θ/n.sub.2).sup.2 }.sup.1/2 !                                                         (5)

where λ₀ is the wavelength of light in a vacuum, n₀ is the refractiveindex of the core layer, n₂ is the refractive index of the fourthcladding layer, θ is the angle formed by the light propagating directionin the core layer and the normal of the multilayer film, and m is aninteger more than 0.

FIG. 28B shows the reflectance of light incident on the waveguide 2413from the waveguide 2412 when the thickness of the fourth cladding layeris varied. In Example 11, the thickness of 0.19 μm when m=1 is used.

In Example 11, the levels of the vertical positions of the waveguides2412 and 2413 are deviated by 0.1 μm. However, in the case where theinfluence of the semiconductor multilayer reflection film is removed asdescribed above, the vertical positions are not necessarily at the samelevel, but the two waveguides only should be arranged so that thecenters of the modes thereof be located substantially at the same level.

The same effect can be obtained when the principle of this example isapplied to other integrated optical control elements.

(EXAMPLE 12)

In Example 12, an element with substantially the same structure as thatof Example 5 is fabricated using another multilayer film fabricationprocess.

First, as shown in FIG. 29A, an Al₀.8 Ga₀.2 As lower cladding layer 2501with a thickness of 1 μm, an Al₀.52 Ga₀.48 As core layer 2502 with athickness of 0.15 μm, an Al₀.8 Ga₀.2 As first upper cladding layer 2503with a thickness of 0.3 μm, a GaAs etching stop layer 2504 with athickness of 10 nm, an Al₀.8 Ga₀.2 As second upper cladding layer 2505with a thickness of 0.7 μm, and a GaAs cap layer 2506 with a thicknessof 0.1 μm are consecutively formed on a semi-insulating GaAs substrate2500 having the (001) plane by MOCVD (metal organic chemical vapordeposition), for example.

Then, a waveguide pattern as shown in FIG. 29A is printed on theresultant wafer by normal photolithography. As shown in FIG. 29B, thesecond upper cladding layer 2505 is selectively etched by wet etching toreach the etching stop layer 2504 by wet etching, so as to form a ridgestructure. This etching process can be easily conducted by wet etchingusing a hydrofluoric acid etchant.

Then, as shown in FIG. 29C, an SiO₂ film 2507 is formed on the resultantwafer to a thickness of about 0.5 μm by sputtering.

Thereafter, as shown in FIG. 30A, a pattern for a concave portion wherea multilayer film is to be formed is formed on the resultant wafer byphotolithography and etching. The pattern width is 3.02 μm. Then, asshown in FIG. 30B, using this pattern, the concave portion havingvertical faces is formed by dry etching such as CAIBE (chemical assistion beam etching) using Cl₂ gas. The concave portion is deep enough toreach the GaAs substrate 2500 as shown in FIG. 30C which is a sectionalview taken along the line a-a' of FIG. 30B.

As shown in FIGS. 30D and 30E, an Al₀.22 Ga₀.78 As layer with athickness of 0.1025 μm, an AlAs layer with a thickness of 0.395 μm, andan Al₀.22 Ga₀.7 As layer with a thickness of 0.103 μm are formed twicein this order inside the concave portion by normal-pressure MOCVD, forexample, so as to form a multilayer film composed of a total of sixlayers. At this stage, the concave portion has an unburied area with awidth of about 1.07 μm.

The unburied area is finally buried with a material having a refractiveindex which is equal to or less than that of the material used for themultilayer film and is larger than n₀ sinθ (where n₀ is the refractiveindex of the first core layer and θ is the angle formed by the lightpropagating direction in the first core layer and the normal of themultilayer film). When the refractive index is smaller than n₀ sinθ,light from the input waveguide is totally reflected from the multilayerfilm. An example of such a material is ZnSe (n⁻ 2.50) grown by MBE(molecular beam epitaxy).

In this example, the refractive index of the final burying layer amongthe plurality of layers constituting the multilayer structure is madelarger than n₀ sinθ (where n₀ is the refractive index of the first layerand θ is the angle formed by the light propagating direction in thefirst layer and the normal of the multilayer structure) and smaller thanthose of the other layers of the multilayer structure. Thus, a precisiondeviation in the fabrication process can be absorbed by the finalburying layer. In other words, when the layers of the multilayerstructure are sequentially formed, the variation in the width of theconcave portion results in the variation in the thickness of the finalburying layer. The final burying layer therefore has a variation in itsthickness substantially equal to the size variation in the fabricationprocess. According to the layer structure of the present invention,however, the variation in reflectance due to the variation in thethickness of the final burying layer can be suppressed.

Alternatively, the refractive index of the thickest layer among theplurality of layers constituting the multilayer structure may be madelarger than n₀ sinθ and smaller than those of the other layers of themultilayer structure. In this case, the same effect as that describedabove can be obtained by appropriately controlling the thickness of thethickest layer in consideration of an expected variation in thefabrication process. With the above-described structure and fabricationmethod, an element with high production yield can be realized withoutrequiring high precision in the fabrication process.

FIG. 31 shows the reflectance characteristic obtained when the layers inthe concave portion are formed so as to obtain a reflectance of 50%. Thesolid line shows the case where the present invention is applied, i.e.,a ZnSe layer is formed as the final burying layer. The dotted line showsthe case where an Al₀.83 Ga₀.17 As layer is formed as the final buryinglayer. Tables 1A and 1B below show the structures of the former case andthe latter case, respectively.

                  TABLE 1A                                                        ______________________________________                                        Al mole fraction (AlxGa.sub.1-x As)                                                                 Thickness (μm)                                       ______________________________________                                        1      Al.sub.0.22 Ga.sub.0.78 As(n = 3.470)                                                            0.103                                               2      AlAs (n = 2.9477)  0.395                                               3      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.103                                               4      AlAs               0.395                                               5      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.103                                               6      ZnSe               Final burying layer                                 5      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.103                                               4      AlAs               0.395                                               3      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.103                                               2      AlAs               0.395                                               1      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.103                                               ______________________________________                                         Structure of the multilayer structure according to the present invention      (corresponding to the solid line in FIG. 31)                                  *The same number indicates the same layer.                               

                  TABLE 1B                                                        ______________________________________                                        Al mole fraction (AlxGa.sub.1-x As)                                                                 Thickness (μm)                                       ______________________________________                                        1      Al.sub.0.22 Ga.sub.0.78 As(n = 3.470)                                                            0.093                                               2      AlAs (n = 2.9477)  0.395                                               3      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.093                                               4      AlAs               0.395                                               5      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.093                                               6      Al.sub.0.77 Ga.sub.0.23 As (n = 3.100)                                                           Final burying layer                                 5      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.093                                               4      AlAs               0.395                                               3      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.093                                               2      AlAs               0.395                                               1      Al.sub.0.22 Ga.sub.0.78 As                                                                       0.093                                               ______________________________________                                         Structure of the multilayer structure using a substance having a              refractive index similar to that of the multilayer structure as the final     burying layer (corresponding to the dotted line in FIG. 31)              

As is observed from FIG. 31, the process margin width is doubled byusing the low refractive index material as the final burying layer.

Each layer of the multilayer structure in the concave portion should beformed so that the variation in reflectance with the variation in thethickness of the final burying layer falls within a desired reflectancerange. The desired reflectance range should be set within a range wherethe variation in reflectance of the multilayer structure with thevariation in the thickness of the final burying layer is small. Thisrange corresponds to portions in the vicinity of peaks of a sinewave ofthe reflectance characteristic curve shown in FIG. 31. In a rangecorresponding to slopes of the sinewave thereof, the variation inreflectance with the variation in thickness of the final burying layeris large. Therefore, conditions under which each layer of the multilayerstructure is formed, for example, the thickness, are set so that thevariation in reflectance of the multilayer structure with the variationin the thickness of the final burying layer falls within the reflectancerange corresponding to the portions in the vicinity of the peaks.

As is observed from FIG. 31, when the thickness of the final buryinglayer (in this case, 1.07 μm) is deviated from the set value by about±0.1 μm, the deviation of the reflectance from a desired value can besuppressed to the range of 40% to 50%.

In Example 12, the effect obtained when the reflectance is set at 50%was described. The effect can also be obtained for any selectedreflectance by selecting a material having a refractive index equal toor less than that of the material used for the multilayer film andlarger than n₀ sinθ.

Further, since the waveguides are formed in one consecutive growthprocess, there arises no deviation in the level of the verticalpositions of the waveguides from each other which tends to arise whenthe waveguides are formed in two consecutive growth processes. Thus,high coupling efficiency is obtained. In dry etching, depth control iscomparatively difficult. However, according to the method of thisexample, control of the depth of the concave portion is not so strictlyrequired. This facilitates the process of the element fabrication.

By the above-described method, a high-performance integrated opticalcontrol element with a reflectance of 48% as compared with the designedreflectance of 50% and a waveguide loss as low as about 0.5 dB wasfabricated with good reproducibility.

(EXAMPLE 13)

In Example 13, the optical branching waveguide (integrated opticalcontrol element) of Example 11 is applied to an optical integratedcircuit device.

FIG. 32 shows an optical integrated circuit device using a 2×2 opticalbranching waveguide 2700 for coherent detection shown in Example 11. ADFB laser 2702 is connected to a waveguide 2413 of the optical branchingwaveguide 2700 so that the laser light output end face of the DFB laser2702 is in contact with the end face of the waveguide 2413. Thus, laserlight (local oscillation light) from the DFB laser 2702 is introduced tothe waveguide 2413. An optical fiber 2701 transmitting signal light isconnected to and in contact with the end face of a waveguide 2414, sothat the signal light is introduced to the waveguide 2414. The localoscillation light and the signal light are branched to waveguides 2412and 2415. Light receiving elements (not shown) are disposed at the endfaces of the waveguides 2412 and 2415 for coherent detection.

In Example 13, since the integrated optical control element can outputlight in any direction, the packaging of the optical element can beefficient compared with conventional elements. This makes it possible toreduce the size and weight of the optical integrated circuit device.

(EXAMPLE 14)

FIG. 33 shows an optical integrated circuit element of Example 14.

The optical integrated circuit element of this example has an opticalinput 2802 composed of a tapered waveguide. An optical fiber 2800a isconnected to this element via an external lens 2801. More specifically,a light intensity modulation signal transmitted from outside through theoptical fiber 2800a is introduced to the optical integrated circuitelement via the lens 2801.

The optical input 2802 composed of a tapered waveguide provides aneffect of minimizing the reduction of the coupling efficiency even whenthe positional relationship between the optical fiber 2800a and thewaveguide portion of the optical integrated circuit element is deviated.For example, the positional relationship is deviated by severalmicrometers by a temperature change even when the optical fiber and thewaveguide are firmly fixed to each other. In such a case, the reductionof the coupling efficiency can be prevented by having the taperedwaveguide. The structure of such a tapered waveguide may have astructure described in A. Fenner Milton, et al., "Mode Coupling inOptical Waveguide Horns", IEEE, J.Q. EQE-13, p 828 (1977), for example.

An optical amplifier 2803a is disposed downstream of the optical input2802 for amplifying the signal light because the signal light inputthrough the optical fiber 2800a becomes weak after long-distancetransmission. The optical amplifier may have a structure described inKitamura, et al., "Polarization Insensitive Semiconductor OpticalAmplifier Array Grown by Selective MOVPE", 1993 Electronic InformationCommunication Association Fall Convention 4-180, for example.

An integrated optical control element 2804a is disposed downstream ofthe optical amplifier 2803a for dividing the signal light into two. Anoptical detector 2805 is disposed downstream of the integrated opticalcontrol element 2804a for converting one of the two beams divided by theintegrated optical control element 2804a into an electric signal. Thesignal is used as a monitor for demodulating a data signal ormaintaining the communication system.

The other of the divided beams is sent to an optical amplifier 2803b foramplifying the beam which becomes weak by the division. The amplifiedbeam is then divided into two by an integrated optical control element2804b. One of the divided beams is output from the waveguide portion,converged by a microlens 2806, and then coupled to an optical fiber2800b. The microlens may have a structure described in Shimada, et al.,"Microlens Fabricated by the Planner Process(V)-Integration with InPLaser Diode", The 55th Applied Physics Association Lecture MeetingExtended Abstract, p-908, for example.

The other of the divided beams is again amplified by an opticalamplifier 2803c, and then divided into two by an integrated opticalcontrol element 2804c. The two divided beams are converged bymicrolenses 2806 and then received by optical fibers 2800c and 2800d,respectively. The signal light received by the optical fibers 2800c and2800d is transmitted to other optical integrated circuit elements (notshown), for example.

As described above, in Example 14, small-size integrated optical controlelements with high stability against temperature change are used incombination with other optical elements, for example, passive waveguidessuch as external modulators, LEDs, and spot converters. Thus, a systemwith low cost and high stability can be provided for an opticaltransmission system for CATV, for example.

Thus, according to the integrated optical control element, the methodfor fabricating the same, and the optical integrated circuit element andthe optical integrated circuit device using the same of the presentinvention, the following effects can be obtained.

(1) By using the multilayer reflection film formed on the vertical faceof the waveguides, an integrated optical control element as a very smallmulti/demultiplexer with a size of the wavelength width can be realized.Also, an element with high stability where the characteristic littlevaries with the variations in wavelength used and operating temperatureof the element.

(2) By using the fabrication process described above, an integratedoptical control element having the multilayer reflection film formed onthe vertical face can be fabricated with high production yield and highreproducibility.

(3) By applying the integrated optical control element to amulti-function semiconductor laser element, an optical integratedcircuit element for coherent detection, and an optical integratedcircuit element for multi-branching, an element which is smaller andmore stable in characteristics than conventional elements can berealized. Also, the positions of the outputs of a package can be freelydesigned.

(4) The coupling efficiency is improved by having a structure where afirst waveguide is composed of a first core layer which becomes a corefor guiding light and a first cladding layer located below the firstcore layer and having a refractive index smaller than that of the firstcore layer, a second waveguide is composed of a second core layer and asecond cladding layer located below the second core layer and having arefractive index smaller than that of the second core layer, themultilayer film is formed below the second cladding layer, and thethickness of the second cladding layer is set at a value equal to ormore than 0.3 times and equal to or less than 2.1 times of the distancewhere the beam intensity at the first waveguide decreases from themaximum value to a value of exp(-2) times the maximum value in the firstcladding layer.

(5) The variation in reflectance can be suppressed by having a structurewhere a first waveguide is composed of a first core layer which becomesa core for guiding light and a first cladding layer located below thefirst core layer and having a refractive index smaller than that of thefirst core layer, a second waveguide is composed of a second core layerand a second cladding layer located below the second core layer andhaving a refractive index smaller than that of the second core layer,the multilayer film is formed below the second cladding layer, and thethickness of the second cladding layer is set at a value obtained by theequation (5) described in Example 11.

(6) Light can be easily coupled at a step by fabricating the integratedoptical control element by the method including the steps of: formingthe step on a substrate to form a first substrate region and a secondsubstrate region with different heights, and consecutively forming afirst waveguide layer structure, and a multilayer structure located nearthe boundary of the first substrate region and the second substrateregion which is arranged to be perpendicular to the first waveguidelayer structure, and a second waveguide layer structure arranged to beparallel to the first waveguide layer structure. The first waveguidelayer structure includes a first layer as a core for guiding light andsecond and third layers located above and below the first layer,respectively, each of the second and third layers having a refractiveindex smaller than that of the first layer. The multilayer structure hasone or more layers with a different refractive index from thesurrounding equivalent refractive index. The second waveguide layerstructure includes a fourth layer as a core for guiding light and fifthand sixth layers located above and below the fourth layer, respectively,each of the fifth and sixth layers having a refractive index smallerthan that of the fourth layer. The first waveguide layer structure, themultilayer structure and the second waveguide layer structure are formedso that the levels of the vertical positions of the first layer at thefirst substrate region and the fourth layer at the second substrateregion are identical to each other. At this time, the radiation loss canbe reduced by controlling the thicknesses of the layers.

(7) The optical integrated circuit element for coherent detectionincludes a plurality of waveguides and a laser for local oscillation. Bydirectly forming a multilayer structure on an end face of the waveguidein the integrated optical control element for guiding signal light inputfrom outside, with no layer between the end face of the waveguide andthe multilayer structure, the reflection efficiency and couplingefficiency increases, and the variation in reflectance and transmittancecan be minimized.

(8) A waveguide includes a first layer which becomes a core for guidinglight and second and third layers located above and below the firstlayer and having a refractive index smaller than that of the firstlayer. A concave portion at least extending through the second or thirdlayer located opposite to a substrate with respect to the first layer isformed so as to divide the waveguide into a first waveguide and a secondwaveguide. Layers are consecutively formed inside the concave portion toform a multilayer structure in the concave portion. Thus, the resultanttwo waveguides are separated by the concave portion, and no deviation inthe level of the vertical positions of the two waveguide from each otheroccurs.

(9) According to the present invention, the refractive index of afinally-formed layer of the multilayer structure is made larger than n₀sinθ (where n₀ is the refractive index of the first layer of themultilayer structure and θ is the angle formed by the light propagatingdirection in the first layer and the normal of the multilayer structure)and smaller than those of the other layers of the multilayer structure.Thus, a precision deviation in the fabrication process can be absorbedby the finally-formed layer. More specifically, when the layers of themultilayer structure are sequentially formed, the variation in the widthof the concave portion results in the variation in the thickness of thefinally-formed layer, resulting in a variation in reflectance ortransmittance of the multilayer structure. According to the layerstructure of the present invention, however, such a variation inreflectance or transmittance of the multilayer structure can besuppressed.

Alternatively, the refractive index of the thickest layer of themultilayer structure may be made larger than n₀ sinθ and smaller thanthose of the other layers. In this case, the same effect as thatdescribed above can be obtained by appropriately controlling thethickness of the thickest layer in consideration of an expectedvariation in the fabrication process. With the above-described structureand fabrication method, an element with high production yield can berealized without requiring high precision in the fabrication process.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. An integrated optical control elementcomprising:a first waveguide for allowing light incident from outside topropagate therein; a multilayer structure for allowing the light whichhas propagated in the first waveguide to be incident thereon and fortransmitting the light therethrough or reflecting the light therefrom,the multilayer structure including at least one layer having arefractive index different from an equivalent refractive index of aregion with which the at least one layer is in contact; and a secondwaveguide for receiving at least part of the light transmitted throughor reflected from the multilayer structure, wherein each of the firstwaveguide and the second waveguide is a channel type waveguide in whicha propagating light is confined in a lateral direction of the waveguide.2. An integrated optical control element comprising:a first waveguidefor allowing light incident from outside to propagate therein; amultilayer structure for allowing the light which has propagated in thefirst waveguide to be incident thereon and for transmitting the lighttherethrough or reflecting the light therefrom, the multilayer structureincluding at least one layer having a refractive index different from anequivalent refractive index of a region with which the at least onelayer is in contact; and a second waveguide for receiving at least partof the light transmitted through or reflected from the multilayerstructure, wherein the multilayer structure includes two or more layershaving different refractive indexes, each of the two or more layers havea thickness expressed by:

    (2m+1)λ.sub.0 /(4n cos θ)

where λ₀ is a wavelength of light in a vacuum, θ is an angle formed by alight proceeding direction and a normal of a layer, n is a refractiveindex thereof, and m is an integer more than
 0. 3. An integrated opticalcontrol element according to claim 2, wherein the mode of the lightincident on the multilayer structure from the first waveguide is a TMmode, and the angle θ is 45° or more.
 4. An integrated optical controlelement comprising:a first waveguide for allowing light incident fromoutside to propagate therein; a multilayer structure for allowing thelight which has propagated in the first waveguide to be incident thereonand for transmitting the light therethrough or reflecting the lighttherefrom, the multilayer structure including at least one layer havinga refractive index different from an equivalent refractive index of aregion with which the at least one layer is in contact; and a secondwaveguide for receiving at least part of the light transmitted throughor reflected from the multilayer structure, wherein the first waveguideincludes a first core layer which becomes a core for guiding the light,and a first cladding layer located below the first core layer and havinga refractive index smaller than that of the first core layer; the secondwaveguide includes a second core layer and a second cladding layerlocated below the second core layer and having a refractive indexsmaller than that of the second core layer; the multilayer structure islocated below the second cladding layer; and the thickness of the secondcladding layer is set at a value equal to or more than 0.3 times andequal to or less than 2.1 times of a distance where a beam intensity atthe first waveguide decreases from a maximum value to a value of exp(-2)times the maximum value in the first cladding layer.
 5. An integratedoptical control element comprising:a first waveguide for allowing lightincident from outside to propagate therein; a multilayer structure forallowing the light which has propagated in the first waveguide to beincident thereon and for transmitting the light therethrough orreflecting the light therefrom, the multilayer structure including atleast one layer having a refractive index different from an equivalentrefractive index of a region with which the at least one layer is incontact; and a second waveguide for receiving at least part of the lighttransmitted through or reflected from the multilayer structure, whereinthe first waveguide includes a first core layer which becomes a core forguiding the light, and a first cladding layer located below the firstcore layer and having a refractive index smaller than that of the firstcore layer, the second waveguide includes a second core layer and asecond cladding layer located below the second core layer and having arefractive index smaller than that of the second core layer, themultilayer structure is located below the second cladding layer, and thethickness of the second cladding layer is a value obtained from:

    mλ.sub.0 / 2n.sub.2 {1-(n.sub.0 sin θ/n.sub.2).sup.2 }.sup.1/2 !

where λ₀ is a wavelength of light in vacuum, n₀ is a refractive index ofthe first core layer, n₂ is a refractive index of the second claddinglayer, θ is an angle formed by a light propagating direction in thefirst core layer and a normal of the multilayer structure, and m is aninteger more than
 0. 6. An optical integrated circuit element comprisinga substrate and a plurality of optical elements formed integrally on thesubstrate, the optical elements including at least one integratedoptical control element of claim
 1. 7. An optical integrated circuitelement according to claim 6, further comprising a multi-functionsemiconductor laser for supplying light to the at least one integratedoptical control element, which outputs a plurality of coherent laserbeams.
 8. An optical integrated circuit element according to claim 6,wherein the optical integrated circuit element is an optical integratedcircuit element for coherent detection and further comprises a laser forlocal oscillation for supplying light to the at least one integratedlight control element.
 9. An optical integrated circuit elementaccording to claim 6, wherein the at least one integrated opticalcontrol element is a light demultiplexing element for dividing one inputbeam into a plurality of output beams.
 10. An optical integrated circuitelement according to claim 8, wherein the optical integrated circuitelement further comprises a plurality of waveguides, and the multilayerstructure is directly formed on a waveguide end face in the at least oneintegrated optical control element for guiding signal light input fromoutside.
 11. An integrated optical control element comprising:awaveguide including a first layer as a core for guiding light and secondand third layers located above and below the first layer, respectively,each of the second and third layers having refractive indexes smallerthan that of the first layer; a concave portion formed in a portion ofthe waveguide, the concave portion extending through at least the firstlayer and either one of the second and third layer which is closer tothe substrate, the concave portion dividing the waveguide into a firstwaveguide portion and a second waveguide portion; and a multilayerstructure formed inside the concave portion by sequentially forming aplurality of layers inside the concave portion, wherein the waveguide isa channel type waveguide in which the propagating light is confined in alateral direction of the waveguide.
 12. An integrated optical controlelement comprising:a waveguide including a first layer as a core forguiding light and second and third layers located above and below thefirst layer, respectively, each of the second and third layers havingrefractive indexes smaller than that of the first layer; a concaveportion formed in a portion of the waveguide, the concave portionextending through at least the first layer and either one of the secondand third layer which is closer to the substrate, the concave portiondividing the waveguide into a first waveguide portion and a secondwaveguide portion; and a multilayer structure formed inside the concaveportion by sequentially forming a plurality of layers inside the concaveportion, wherein the refractive index of a layer formed finally amongthe plurality of layers constituting the multilayer structure is largerthan a refractive index obtained from n₀ sinθ where n₀ is a refractiveindex of the first layer and θ is an angle formed by a light propagatingdirection in the first layer and a normal of the multilayer structureand is smaller than that of the other layer or layers of the multilayerstructure.
 13. An integrated optical control element comprising:awaveguide including a first layer as a core for guiding light and secondand third layers located above and below the first layer, respectively,each of the second and third layers having refractive indexes smallerthan that of the first layer; a concave portion formed in a portion ofthe waveguide, the concave portion extending through at least the firstlayer and either one of the second and third layer which is closer tothe substrate, the concave portion dividing the waveguide into a firstwaveguide portion and a second waveguide portion; and a multilayerstructure formed inside the concave portion by sequentially forming aplurality of layers inside the concave portion, wherein the refractiveindex of the thickest layer among the plurality of layers constitutingthe multilayer structure is larger than a refractive index obtained fromn₀ sinθ where n₀ is a refractive index of the first layer and θ is anangle formed by a light propagating direction in the first layer and anormal of the multilayer structure and smaller than a refractive indexof the other layer or layers of the multilayer structure.
 14. An opticalintegrated circuit device comprising a plurality of optical elements incombination, wherein at least one of the plurality of optical elementsis the integrated optical control element of claim
 1. 15. An opticalintegrated circuit device according to claim 14, further including amulti-function laser for supplying light to the integrated opticalcontrol element, which outputs a plurality of coherent laser beams. 16.An optical integrated circuit device according to claim 14, wherein theoptical integrated circuit device is an optical integrated circuitdevice for coherent detection and further includes a laser for localoscillation for supplying light to the integrated optical controlelement.
 17. An optical integrated circuit device according to claim 16,wherein the optical integrated circuit device is an optical integratedcircuit device for coherent detection where local oscillation light andsignal light are input from outside to the integrated optical controlelement including a plurality of waveguides, and the multilayerstructure is directly formed on a waveguide end face in the integratedoptical control element for guiding the signal light.